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D.I.Y. amp

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Re: D.I.Y. amp

PostPosted: Fri Mar 18, 2016 11:41 am
by Hugh Robjohns
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

Re: D.I.Y. amp

PostPosted: Fri Mar 18, 2016 12:29 pm
by Folderol
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!

Re: D.I.Y. amp

PostPosted: Fri Mar 18, 2016 1:15 pm
by Hugh Robjohns
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

Re: D.I.Y. amp

PostPosted: Fri Mar 18, 2016 5:32 pm
by Folderol
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.

Re: D.I.Y. amp

PostPosted: Fri Mar 18, 2016 5:43 pm
by Hugh Robjohns
Doesn't sound like headroom margin is much of an issue then. ;)

H

Re: D.I.Y. amp

PostPosted: Wed Mar 23, 2016 3:26 pm
by Martin Walker
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


Martin

Re: D.I.Y. amp

PostPosted: Wed Mar 23, 2016 6:55 pm
by Folderol
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 :)

Re: D.I.Y. amp

PostPosted: Thu Mar 24, 2016 8:39 pm
by Martin Walker
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:


Martin

Re: D.I.Y. amp

PostPosted: Thu Mar 24, 2016 10:11 pm
by Folderol
Bar Steward Image

Re: D.I.Y. amp

PostPosted: Fri Mar 25, 2016 12:54 am
by blinddrew
I've read all of this thread and found it fascinating whilst simultaneously not understanding a word.

I like the pictures.

Re: D.I.Y. amp

PostPosted: Sat Mar 26, 2016 12:19 am
by Folderol
blinddrew wrote:I've read all of this thread and found it fascinating whilst simultaneously not understanding a word.
Oh...


I like the pictures.
Well that's nice :)

Re: D.I.Y. amp

PostPosted: Sun Mar 27, 2016 4:45 pm
by Martin Walker
blinddrew wrote:I like the pictures.

I see quite a few people standing for hours in newsagents who claim the same thing :roll:


Martin

Re: D.I.Y. amp

PostPosted: Mon Mar 28, 2016 12:54 am
by zenguitar
Martin Walker wrote:
blinddrew wrote:I like the pictures.

I see quite a few people standing for hours in newsagents who claim the same thing Image


Martin

Oi know where you live Martin...

It was either Farmers' Weekly or Guns & Ammo they were browsing ;)

Andy :beamup:

Re: D.I.Y. amp

PostPosted: Mon Mar 28, 2016 2:56 am
by Folderol
... and there was I thinking you'd come to comment on the latest update :frown:

Oh well {sigh}

Re: D.I.Y. amp

PostPosted: Mon Mar 28, 2016 3:56 pm
by Martin Walker
Folderol wrote:Hi Martin,

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

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!

This is excellent news - hum levels are generally far lower with toroidal transformers (due to their weaker radiated magnetic field) than EI-frame designs.

OK Folderol?


Martin