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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!
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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 :)
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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:
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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.
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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:
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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 ?
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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!
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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.
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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-)
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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:
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Re: D.I.Y. amp

Postby Hugh Robjohns » Fri Mar 18, 2016 11:41 am

Folderol wrote:The input isolation is switchable so I can either use it unbalanced which gives a greater overload margin or float the socket grounds


I presume the buffer after the transformer is the reasons for the overload margin compromise... but why does it need a buffer there at all? Could the transformer secondary not just feed straight into the rest of the amp?

H
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Re: D.I.Y. amp

Postby Folderol » Fri Mar 18, 2016 12:29 pm

The transformer is working in the so-called 'zero field mode' The op-amp is configured as a virtual earth which means the transformer is acting as a current generator rather than a voltage source. This makes the transformer itself look pretty much like a short circuit (hence the input resistors). In this configuration the transformer is effectively immune to LF saturation and displays remarkable linearity over the whole audio range. If I get cruel and poke a 100Hz square wave into it the output remains flat topped!
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Re: D.I.Y. amp

Postby Hugh Robjohns » Fri Mar 18, 2016 1:15 pm

Folderol wrote:In this configuration the transformer is effectively immune to LF saturation and displays remarkable linearity over the whole audio range.

Ah ha! Yes -- I hadn't clocked zero-field mode configuration... So, you've exchanged the compromised LF and linearity issues of an inexpensive input transformer for a compromised headroom margin! ;) Fair enough!

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Re: D.I.Y. amp

Postby Folderol » Fri Mar 18, 2016 5:32 pm

Hugh Robjohns wrote:
Folderol wrote:In this configuration the transformer is effectively immune to LF saturation and displays remarkable linearity over the whole audio range.

Ah ha! Yes -- I hadn't clocked zero-field mode configuration... So, you've exchanged the compromised LF and linearity issues of an inexpensive input transformer for a compromised headroom margin! ;) Fair enough!

H
It was actually a no-brainer. The ones I used were end-of-line and just £8. Also, with an input sensitivity of 0dBm for full power, and +-12V supply to the amps that still gives over 20dB margin.
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Re: D.I.Y. amp

Postby Hugh Robjohns » Fri Mar 18, 2016 5:43 pm

Doesn't sound like headroom margin is much of an issue then. ;)

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Re: D.I.Y. amp

Postby Martin Walker » Wed Mar 23, 2016 3:26 pm

Folderol wrote:First the bad news. There is a very faint buzz (rather than hum) slightly more from the LH channel, which is closest to the mains TX - rather points to radiated field. I'll have a go a screening but am not too optimistic. Hum and noise measures at -80dB, but that is relative to 50W, and I'm sitting just over a metre from my nearfields :roll:


Sounds like a job for a mu-metal screen Image


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Re: D.I.Y. amp

Postby Folderol » Wed Mar 23, 2016 6:55 pm

Hi Martin,

Well it's nice to know someone reads back over this missive ;)

However, that paragraph refers to the original setup with a 'standard' mains transformer. This had been chosen because it was double insulated and allowed the use of a semi-floating ground. The buzz was being picked up by the late amplifier stages, not any part of the preamp.

The new transformer is a toroidal one, and now I have the input isolators I use a 'hard' ground. The hum level from the speakers is now much lower than the mechanical hum from the transformer!

Oh, and those 'cheap' input transformers do in fact have mumetal cans :)
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Re: D.I.Y. amp

Postby Martin Walker » Thu Mar 24, 2016 8:39 pm

Folderol wrote:Hi Martin,

Well it's nice to know someone reads back over this missive ;)

Hi Folderol!

I insist on reading all your posts before I can get off to sleep at night :beamup:


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Re: D.I.Y. amp

Postby Folderol » Thu Mar 24, 2016 10:11 pm

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Re: D.I.Y. amp

Postby blinddrew » Fri Mar 25, 2016 12:54 am

I've read all of this thread and found it fascinating whilst simultaneously not understanding a word.

I like the pictures.
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