You are here

D.I.Y. amp

Customising, building or repairing your own gear? Need help with acoustic treatment or soundproofing? Ask away…

D.I.Y. amp - correction :(

Postby Folderol » Sun Oct 13, 2013 5:13 pm

Gah! silly mistake on the schematic. Sorted now - sorry :blush:
User avatar
Folderol
Jedi Poster
Posts: 10877
Joined: Sat Nov 15, 2008 1:00 am
Location: The Mudway Towns, UK
Yes. I am that Linux nut.
Onwards and... err... sideways!

D.I.Y. amp - Onwards and Upwards

Postby Folderol » Wed Nov 20, 2013 1:16 am

At last I've been able to get back to work on this project... for a while!

First I've done some relatively minor improvements to the logic section. I've also finalised the PSU, using decent voltage regulators. After this, I went through all the drawings updating them and making them clearer.

The ones that are significantly changed are:

http://www.musically.me.uk/drawings/PSU.pdf
http://www.musically.me.uk/drawings/Logic.pdf

There is also a better picture of the bare logic PCB:

http://www.musically.me.uk/images/Logic_Bare.JPG


And now the new stuff!

I've completed the design of the preamp. It's not fully built and might need a few tweaks but the attenuator/volume control is done and tone controls and filters have all been tried out in breadboard form and seem OK.

The input attenuator, which is already in place in the amp with just a basic buffer following, gives approximately 10dB steps when fed from a low impedance source - the volume control being an integral part of the attenuator ladder. I wanted this so that whatever overall level I was listening at, I would always have a wide range for the volume control itself. A side benefit I've found is that when checking fade-outs of music I'm working on, I can step through the attenuator increasing the output in sensible steps while listening for discontinuities etc.

The tone controls are pretty conventional, but the centre frequency is a little lower than usual at about 800Hz, which seems nicer to me.

The crossover filters use fixed capacitors and switched resistor chains to cover the range 70 - 125Hz. The pattern of resistors I've used gives quite good precision relatively easily with preferred values. The caps are 1% and so are the higher value resistors (the rest are 2%). I probably didn't need to go that far, but I like the idea of a really close match of all 4 frequency determining sections. Sub drive is summed from the low pass sections of both L & R filters. Gain is set at +- 12dB which I hope will be adequate. If and when I get a sub unit I would expect to set these controls up then leave then severely alone.

If the preamp is switched for simple stereo, there is an overall phase inversion. I could have messed around using the first stage of the filter as a plain inverting amplifier instead, but that would have required more complex switching, and I really don't think it's important enough. I don't know if anyone else can detect absolute phase, but I'm pretty sure I can't!


As well as a drawing for the preamp, I've also put up one showing the ground routing which should make sure I have no internal ground loops. Although the logic is powered separately it has to be tied to the main ground in order to measure the various voltages.

http://www.musically.me.uk/drawings/PreAmp.pdf
http://www.musically.me.uk/drawings/Grounds.pdf


The relay control is done in principle but not installed yet (not much point until everything else is there). K1,2 & 6 could have all been operated by the same driver, but I had spares in the chip. Currently K4 and K5 are directly wired to the 24V supply. Drawing is here:

http://www.musically.me.uk/drawings/Control.pdf


Headphone output hasn't been done at all yet. I'm thinking of again using switched resistors to give the phones a fairly low impedance source along with a range of levels so that the apparent output from them can be set similar to that of the speakers. The logic selects phones or speakers, not both. I can't think of any reason for wanting both at the same time, and in the past, I have been caught out listening on phones and not realising the speakers were on as well!
User avatar
Folderol
Jedi Poster
Posts: 10877
Joined: Sat Nov 15, 2008 1:00 am
Location: The Mudway Towns, UK
Yes. I am that Linux nut.
Onwards and... err... sideways!

D.I.Y. amp - a little set-back

Postby Folderol » Tue Dec 03, 2013 11:20 pm

Things had been going well, perhaps too well.

The preamp board is built, but as there is a lot of gain around the opamps I've been populating it stage at a time looking carefully for any hint of instability. I'd just completed the tone controls and had needed, as I expected, to add a couple of extra caps to kill a bit of ringing when set for maximum treble. Controls were behaving really nicely with just the right amount of cut and lift.

By chance I shorted a makeshift attenuator I had hanging on the inputs resulting in a massive overload that pushed the amp so far into limiting it went asymmetric. The good news is that the logic (now also fully connected) picked up the effective DC offset and disconnected the output relays as it should. Once I'd pulled the input, the relays snapped back again quite correctly.

The bad news is that there was now about 1/2 volt of really weird HF crap on the outputs. A quick check all round and voltages all seemed normal, so was the current on the output stages. Switched off, did some more checks and had a good look around, then switched on again and everything was back to normal. Deliberately overloaded the input and the same thing happened. What's more I found I could set this off with just a high amplitude pulse on the input. Now I was worried!

Disconnecting the preamp and going straight into the main amp inputs cured the problem, so I started to think it was a preamp stability problem, even though that seemed really unlikely. This actually tuned out to be a red herring. Eventually I thought about putting the scope on the supply rails. Everything was fine ... till I looked at the -12V rail which was going absolutely nuts!

If you look at the PSU drawing, you'll see the supplies are symmetrical, so why was just the negative regulator misbehaving? I carefully checked the wiring for silly mistakes and examined the decoupling capacitors but everything seemed OK. Then I just tapped an additional cap across the -12V line to ground. The oscillations instantly stopped. Thinking I had maybe a dud cap I replaced it, but the fault was still there. However, additional caps on just the negative line cured the fault.

Confession time!

I'd looked up the LM7812 data sheet to confirm the decoupling I needed, which was >= 330nF on the input and >= 100nF on the output, then just repeated that for the LM7912. Just a mirror image, right? Well, wrong! When I thought to look up that one's data sheet it had >= 2u2 on the input and >= 1u on the output.

I've left the 100nF caps in place and now added 10u caps on both + and - outputs. The thing is now perfectly stable no matter what I hit the input with. Only a couple of days wasted. Phew!
User avatar
Folderol
Jedi Poster
Posts: 10877
Joined: Sat Nov 15, 2008 1:00 am
Location: The Mudway Towns, UK
Yes. I am that Linux nut.
Onwards and... err... sideways!

D.I.Y. amp - getting there!

Postby Folderol » Thu Dec 19, 2013 1:09 am

Well first off I found another 'gotcha'. Wiring the sub signal and power channels together was a mistake. If the sub channel develops an overdrive fault it will switch off of course. This presents two problems. The first is that the normal speakers will now be handling the bass (and in some future setup that might not be advisable). The second is that with the routing changed, there will no longer be the signal condition that caused the fault, so after a second or two it will be reconnected. Rinse and repeat!

The advantage of having spare I/O comes to the rescue. I already had separate drivers for the relays, so all I needed to do was take another line back to the controller and make a minor software change. Now, when selected, the sub signal routing remains in place if there is an output fault registered, and just the output is disconnected with an appropriate warning on the display.

The following drawings have been updated to reflect this:
http://www.musically.me.uk/drawings/Logic.pdf
http://www.musically.me.uk/drawings/Control.pdf


A minor change has also been made to the preamp as I found there was an overall 3dB 'hump' at the crossover point when the sub channel was enabled. At the moment I've just lowered the value of two resistors to reduce the 'Q' of the filters, but I'm considering making this user adjustable too so that there is a better match to whatever speakers are employed.

New drawing is:
http://www.musically.me.uk/drawings/PreAmp.pdf


The board itself (without the 56pF caps I added later) is:
http://www.musically.me.uk/images/PreAmp_Bare.JPG


The idea of switched resistors for the headphone gain matching has been tested and seems to work reasonably well, but still isn't installed in the amp yet. I don't exactly have a huge range of phones, so there's a fair bit of guesswork involved, and (no doubt) later adjustment.

Finally here is a picture of the current state of the chassis:
http://www.musically.me.uk/images/Chassis_2.JPG


You can clearly see the new voltage regulators above the main reservoir caps. Its mounting plate is also the heatsink for the 4 voltage regulators, and is easily removable (hence the two cut-outs in the PCB. The logic is also pretty thoroughly shielded from the rest of the amp, with just very short leads to the display. Also, all the signal connections between the preamp board, controls & main amp are fully screened, and I did a quick crosstalk check which seemed to indicate better than -50dB @ 10kHz, and buried in the noise @ 1kHz. The screened wires to the input sockets tuck nicely into the groves in the side frame!
User avatar
Folderol
Jedi Poster
Posts: 10877
Joined: Sat Nov 15, 2008 1:00 am
Location: The Mudway Towns, UK
Yes. I am that Linux nut.
Onwards and... err... sideways!

D.I.Y. amp - a bit of a hiatus

Postby Folderol » Mon Jan 13, 2014 8:17 pm

Well, we've had a major shunt around at work, (where all this fun stuff gets done in my free time) and with the holiday period as well, things have rather come to a standstill :frown:

Our office is a very old converted granary. Since the shuffle I'm now one floor up in the building. which means more light and some nice countryside views.

Anyway, all my bits and pieces - gathered over many years - are in place, and I've actually got a little more floorspace than I had before. The bench itself is my pride and joy and has followed me around for about 12 years, even though the bosses keep saying I should have something more 'modern' (i.e. smaller).

So for a change instead of drawings here is the workshop.

P.S.
A small safety point. Although those side frames are metal, they are completely isolated - not even connected to each other, and everything else is heavy duty plastic coated chipboard, including the drawer unit.
User avatar
Folderol
Jedi Poster
Posts: 10877
Joined: Sat Nov 15, 2008 1:00 am
Location: The Mudway Towns, UK
Yes. I am that Linux nut.
Onwards and... err... sideways!

DIY amp - lifting her skirts

Postby Folderol » Sat Feb 15, 2014 2:01 am

Sometimes you get a feeling that something isn't quite right and even though the amp seems to be behaving perfectly I've had that nagging doubt for a while. In an earlier writeup I mentioned that a severe overload, well into clipping, would make the amplifier output become asymmetric, giving an effective DC offset. It was this that was bothering me.

Eventually I found time to drag the amp into the workshop again and do some more detailed testing. What I discovered was that the offset developed far more rapidly than I would have expected, almost instantly in fact, and I didn't remember seeing that in my earliest trials. Referring back to my original drawings, apart from some minor adjustments of values, the only significant change in the amp itself was the addition of the cap across the 220 ohm resistor feeding the emitter of the BC550 pre-driver transistor. OK, so if it was charging up for some reason that would certainly do it.

Disconnecting the cap on the left channel and comparing it with the right one, proved this was the case. The precise cause took a little while to work out and was due to a little cascade of issues, originating in a couple of resistors who's only function was protection! The 100 ohm resistors going from the drivers to the bases of the output transistors are there to ensure that under overload conditions the driver current can't exceed a safe level, but this has a knock-on effect as under these conditions the drivers saturate and stop behaving like transistors with the input resistance of the drivers' bases rapidly dropping.

On the positive half-cycle this simply means the 2k2 pull-up resistor determines the current into the upper transistor, while on the negative half-cycle the current is determined by the pre-driver and it's emitter resistor, which could be near ten times as much. It is this that gives the asymmetric behavior, but this does no harm as everything is working well within spec.

Adding the capacitor dramatically changes the situation. When the bottom driver saturates the pre-driver briefly has an effective short circuit on its emitter, this means there will be a current pulse that may be well over it's specified maximum while the cap charges up. If it's a maintained overload, the charging of the cap will reduce this pulse quite a lot for each successive half-cycle while at the same time shifting the bias point and causing the rapid DC offset. A worse situation is where high amplitude bursts of signal keep hitting this point, then let the cap discharge again.

So, with some reluctance I've removed the caps, and accepted a slightly higher distortion figure, although I don't hear any difference. Which neatly brings me to some new info.

I have found a reasonably good software spectrum analyzer that uses my sound card. The frequency scale is linear rather than logarithmic but that has some benefits when looking at harmonic distortion. I'm driving the amp with the sound card pushing out about 3dB below its maximum, then attenuating the return from the amp so that peak voltage (fractionally below the clipping point) indicates at 0dB.

The traces are in pairs, first with a 7.5 ohm resistor (two 15s in parallel), then with a dummy 8 ohm speaker load. Note that these are all peak levels, and I'm afraid I've forgotten the formula for adding harmonics to get a total RMS figure.


Anyway, first at 1kHz.

http://www.musically.me.uk/images/Resistor-20db1kHz.jpg
http://www.musically.me.uk/images/Dummy-20db1kHz.jpg

Higher than I would have liked, but almost entirely 2nd harmonic which subjectively adds a tiny amount of 'warmth'.

http://www.musically.me.uk/images/Resistor-10db1kHz.jpg
http://www.musically.me.uk/images/Dummy-10db1kHz.jpg

Distortion becoming more noticeable, and 3rd harmonic climbing, but quite a good match between levels with resistor and a dummy loads.

http://www.musically.me.uk/images/Resistor0db1kHz.jpg
http://www.musically.me.uk/images/Dummy0db1kHz.jpg

Here we have full power. This is right on the verge of the clipping point at just over 60W, but still distortion is acceptable. However there is more of a difference between the resisitive and the dummy loads.


Next we have (not quite) 100Hz. I fiddled the frequency to minimise beat effects with mains.

http://www.musically.me.uk/images/Resistor-20db100Hz.jpg
http://www.musically.me.uk/images/Dummy-20db100Hz.jpg

As expected, the dummy load has a higher 3rd harmonic as its impedance is going to be dropping somewhat at this frequency, thus putting more load on the output transistors.

http://www.musically.me.uk/images/Resistor-10db100Hz.jpg
http://www.musically.me.uk/images/Dummy-10db100Hz.jpg

Again following a similar pattern.

http://www.musically.me.uk/images/Resistor0db100Hz.jpg
http://www.musically.me.uk/images/Dummy0db100Hz.jpg

For these last traces I had to cut the feed to my soundcard by 10dB. I was more than a little annoyed to discover it didn't like the peak signal at 100Hz.

There is more to come!
User avatar
Folderol
Jedi Poster
Posts: 10877
Joined: Sat Nov 15, 2008 1:00 am
Location: The Mudway Towns, UK
Yes. I am that Linux nut.
Onwards and... err... sideways!

D.I.Y. amp - Almost Done

Postby Folderol » Thu Mar 27, 2014 8:41 pm

Well, so much for having more free time these days. I seem to find it harder to get to work on this project.

Anyway we're approaching completion - Woo!

First off, we have the completed phones resistor network. This gives approximately 6dB steps (depending on the actual phones) and for my AKG240s half way up the range gives a pretty close match to my speakers. The choice of values was a multiple compromise between power dissipation, S/C protection, impedance and level. In the drawing there's a greyed out inductor. If I can work out the correct value I'm debating being able to switch this in to approximate the behaviour of speakers.
http://www.musically.me.uk/drawings/Phones.pdf

And now and underside view. The resistor network is pretty obvious in the top LH corner. and the switching relay is in the tiny board to the right and below it. Further across is the Relay drivers board. The decisions about these were all rather piecemeal, and I should probably tidy it all up with a new board for everything. It works, so I probably won't! The three relays at the bottom are for the speakers. I changed from my original choice as these are half the price for the same rating.
http://www.musically.me.uk/images/Underside.JPG

I also decided to add in the ability to change the Q factor of the crossover point and this is shown on an updated pre-amp drawing. You'll see I've started putting in function names and component numbers - there was no point while everything was still in development. This is the sort of work I always hate doing.
http://www.musically.me.uk/drawings/PreAmp.pdf

The front panel layout was settled a long time ago, and I chose control knob positions and sizes to reflect their importance. This is likely to be the biggest headache now, actually producing a front panel overlay. I not only need to get the legends in place, but also to mask over the edges of the display and cover up an 'oops' (a hole in totally the wrong place!)
http://www.musically.me.uk/drawings/Layout.pdf

Some minor alterations to the software. In the picture below, the amp was switched on while there was a strong signal being applied to the RH channel. You can see it on the bargraph, but there is also an 'X' at the end giving visual indication that the output has been disabled. As soon as the level is dropped, the relay will click in and everything behaves normally.
http://www.musically.me.uk/images/Start_Hold.JPG

In a similar way it has been blocked due to a DC offset (I faked it with a resistor on the logic board). Once again if the fault clears everything returns to normal.

http://www.musically.me.uk/images/DC_Hold.JPG

So there you go, only six years to not quite finish :)
User avatar
Folderol
Jedi Poster
Posts: 10877
Joined: Sat Nov 15, 2008 1:00 am
Location: The Mudway Towns, UK
Yes. I am that Linux nut.
Onwards and... err... sideways!

I aint ded yet... as Granny Weatherwax would say.

Postby Folderol » Wed Sep 24, 2014 5:00 pm

Some news!

The amp has been in daily use for quite some time now, and mostly I'm pretty pleased with it. However, watch this space as there are a couple of improvements I'm working on :)

P.S.
I still haven't found somewhere I can get a front panel made that won't require a home re-mortgage :frown:
User avatar
Folderol
Jedi Poster
Posts: 10877
Joined: Sat Nov 15, 2008 1:00 am
Location: The Mudway Towns, UK
Yes. I am that Linux nut.
Onwards and... err... sideways!

D.I.Y. Amp still going strong.

Postby Folderol » Tue Jun 30, 2015 8:01 pm

Wow! It's been some time since the cleaners were in. Look at all the dust round here :frown:

Anyway, I've made two changes to the amp. The first is replacing the mains transformer with a torroidal one. This gives a slightly higher supply voltage and a slighty improved hum level - you need to get your ear right on the speakers to hear it now :)

The second change is that I've fitted (switchable) isolation transformers to the input. This is because there was one particular combination of hardware synths that produced some annoying but inaudible hf. Without the transformers you've got effectively infinite overload margin, with them it's around 40dB.

I'll try to update the drawings soon... ish :tongue:

I still don't have a tidy front panel. Anyone know a place that does bespoke ones, 3U rack size?

I'm thinking the best idea would be a thin-ish one that fits on top of the exisiting thick ally front plate and hides all the defects. It would be held in place by all the pot nuts, and the rack mounting bolts.
User avatar
Folderol
Jedi Poster
Posts: 10877
Joined: Sat Nov 15, 2008 1:00 am
Location: The Mudway Towns, UK
Yes. I am that Linux nut.
Onwards and... err... sideways!

Re: D.I.Y. amp

Postby Folderol » Fri Mar 18, 2016 12:32 am

My, doesn't time fly when you're enjoying yourself?

About 6 months ago I said I'd update the drawings after changing the mains transformer and adding input isolation transformers.

Well here there are ... at last :)

The PSU schematic has hardly changed. There is no longer a need for the pseudo-ground I had before and the fusing is slightly different. Otherwise there just a slightly higher rail voltage.

http://www.musically.me.uk/drawings/PSU.pdf

The input isolation is switchable so I can either use it unbalanced which gives a greater overload margin or float the socket grounds (the chassis is still connected to the rest of the kit) for balanced isolated input. Eventually I'll get around to changing the input sockets to TRS ones... maybe ;)

http://www.musically.me.uk/drawings/Input_Isolation.pdf

I've been very careful with the layout as you can see. The input transformers are just about as far away from the new mains transformer as possible! Also that whole area is now almost completely screened.

http://www.musically.me.uk/images/Chassis_3.JPG

The amp has now run for about 2 years without any significant changes. In that time I've had one switch get a bit noisy, silenced by a quick squirt of contact cleaner.

DC conditions have drifted slightly. Output offset is -7.5mV and -14mV while bias current is 212mA and 195mA for L & R respectively. I didn't bother to check the sub output - it's never been used. Currently there would be no point, my room acoustics are truly horrible!

Oh, and I still don't have a decent front panel :frown:
User avatar
Folderol
Jedi Poster
Posts: 10877
Joined: Sat Nov 15, 2008 1:00 am
Location: The Mudway Towns, UK
Yes. I am that Linux nut.
Onwards and... err... sideways!

Re: D.I.Y. amp

Postby Jumpeyspyder » Fri Mar 18, 2016 1:12 am

Wow, looks great !!

Warning - newbie question ! - whats that mass of resistors, right hand side near front panel ?
User avatar
Jumpeyspyder
Frequent Poster
Posts: 1157
Joined: Fri Jan 20, 2006 1:00 am
Location: Yorkshire

Re: D.I.Y. amp

Postby Dynamic Mike » Fri Mar 18, 2016 1:16 am

Jumpeyspyder wrote:Wow, looks great !!

Warning - newbie question ! - whats that mass of resistors, right hand side near front panel ?
I was wondering about that. It looks like something that's been attacked by Sentinels in the Matrix!
Dynamic Mike
Frequent Poster (Level2)
Posts: 3384
Joined: Sun Dec 31, 2006 1:00 am
'Cause the law don't change another's mind

Re: D.I.Y. amp

Postby Folderol » Fri Mar 18, 2016 1:37 am

It's the crossover frequency switching.

It is in circuit when the sub output is enabled, and the crossover frequency is switchable in 5Hz steps from 70Hz to 125Hz. There are a total of 4 poles, 2 for each input channel. The idea of an accurate 4 pole pot is a non-starter! The crossover slope is also tunable but that is less critical and an ordinary 2 gang pot is good enough for that. Finally the sub gain is variable over a small range, so in theory it should be possible to match just about any combination.

P.S.
It has a use even without a sub speaker. I sometimes switch it to that mode to hear what my latest and greatest sounds like with a poor LF response.
User avatar
Folderol
Jedi Poster
Posts: 10877
Joined: Sat Nov 15, 2008 1:00 am
Location: The Mudway Towns, UK
Yes. I am that Linux nut.
Onwards and... err... sideways!

Re: D.I.Y. amp

Postby Jumpeyspyder » Fri Mar 18, 2016 2:21 am

If I'm understanding that correctly, you've carefully matched groups of resistors that are switched in and out to tune the crossover frequency ?

- If so, that seems some serious dedication 8-)
User avatar
Jumpeyspyder
Frequent Poster
Posts: 1157
Joined: Fri Jan 20, 2006 1:00 am
Location: Yorkshire

Re: D.I.Y. amp

Postby Folderol » Fri Mar 18, 2016 10:07 am

It wasn't actually that difficult. Comparatively easy C/R calculations gave me the wanted values - all 4 elements are identical. Caps and most resistors are 1% and some of the smaller 'padding' ones are 2%. All are E12 preferred values.

The preamp drawing which shows the values and their connection pattern is:
http://www.musically.me.uk/drawings/PreAmp.pdf

P.S.
The hardest part was physically placing all the resistors on the switch :frown:
User avatar
Folderol
Jedi Poster
Posts: 10877
Joined: Sat Nov 15, 2008 1:00 am
Location: The Mudway Towns, UK
Yes. I am that Linux nut.
Onwards and... err... sideways!

PreviousNext