Dual mono or separate transformers for input- and outputstage?

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Hannes-Gregor
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Dual mono or separate transformers for input- and outputstage?

Unread post by Hannes-Gregor »

Having a look at the AP60+ it doesn't use its two transformers to feed both channels separately but to take one for the in- and the other for the output stage. In the description for it it is said that there could be a benefit when driving low efficiency speakers. Are there other benefits? Would there be one if you reconfigure a dual mono like the AP70 to separated power for in- and output stage even if you own effective speakers?

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Re: Dual mono or separate transformers for input- and outputstage?

Unread post by Hannes-Gregor »

No more correction of the previous entry possible, so:

A60+ not Ap60+!

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Re: Dual mono or separate transformers for input- and outputstage?

Unread post by Geoff.R.G »

I would imagine that powering the amplifier front and back produces a better sound than left and right however I would also expect benefits from having four power supplies. Separate left and right, front and back.

The input stage usually runs at a different voltage from the output stage and designing the power supply to do one job very well will always result in superior performance over sharing the supply between two different requirements.
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Hannes-Gregor (Tue Jun 30, 2020 11:24 am)

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Re: Dual mono or separate transformers for input- and outputstage?

Unread post by Hannes-Gregor »

Thank you. So like in the A80's. Or the integrated AP80 which Richard didn't want to build anymore because of all the heat. And then 25V on the input stage and 30V on the output? When I compare the AP20 (25V) with the AP70 (30V) the AP20 is more lively but lacks a bit of bass and substance. A composition of both would be nice.

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Re: Dual mono or separate transformers for input- and outputstage?

Unread post by 29mile »

Interesting post as I am rebuilding my 80’s A60 at the minute with a new PSU built using 2 x 500va transformers giving 2 split rails ie -/0/+ . Part2 of project is to add a second PSU giving 4 transformers but short term I plan to use the 2 split rails to supply front and output stages of both boards. I suspect this is a halfway house compare to 4 rails for front and output stages of each board as per A80’s, TSS etc. I know A80’s use a smaller Va transformer for the front end but assumed it was same voltage as bigger Va output transformer. Perhaps Paul/Tom could enlighten us ?

Ps when I say output stage I mean only the 2 Darlingtons which can be separately powered using the first jumper on the new boards. My a60 only has this single jumper unlike the 3 options available on the new current boards.
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Hannes-Gregor (Tue Jun 30, 2020 11:24 am)

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Re: Dual mono or separate transformers for input- and outputstage?

Unread post by karatestu »

I have found seperate transformer and psu for each channel improves channel separation , detail and stereo effects.

IMO separating each channels psu into two is very worthwhile also. The input stage is in many ways just like an active preamp and just look at the lengths people go to when providing a psu for them (massive hicap transformers and super regulators). The diy Avondale voyager amps I have split the board after the Voltage amplification stage and power the driver transistors on the same psu as the output transistors.

With the input stage you have to be very careful with the voltage as there are limits to what is safe due to the ratings of the transistors. The output transistors are able to safely accept more voltage than the input stage. I think the input stage draws a constant current which is in the mA range. The Avondale input stage draws about 12mA.

The output stage however will be up and down with the music. Having all the ripple and voltage sag caused by the output stage current draw on the psu shared also by the input stages is not ideal. Giving some separation is usually a good thing to do for your ears. I have seen this separation being as simple a diode and resistor in series between the two stages.

Obviously with such a low constant current draw, the input stage does not require as big (VA) transformer as the output stage. But then when you look at the lengths Richard would go to with transformer size (phono3) then maybe bigger is better for the input stage too. Bigger =better transformer regulation and lower impedance.
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Hannes-Gregor (Tue Jun 30, 2020 11:23 am)
DIY inspired by Richard "The Doc" Dunn RIP

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Re: Dual mono or separate transformers for input- and outputstage?

Unread post by Hannes-Gregor »

Thank you all for your thoughts. So the best seems to be feeding each channels with two transformers. But how about one transformer for both in- and one for both output stages? Is there an advantage compared to dual mono? I think there must be a reason why Tomasz and Paul did it that way in the A60+.

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Re: Dual mono or separate transformers for input- and outputstage?

Unread post by 29mile »

karatestu wrote:
Tue Jun 30, 2020 6:24 am
I have found seperate transformer and psu for each channel improves channel separation , detail and stereo effects.

IMO separating each channels psu into two is very worthwhile also. ...But then when you look at the lengths Richard would go to with transformer size (phono3) then maybe bigger is better for the input stage too. Bigger =better transformer regulation and lower impedance.
Very helpful post. The TSS I owned had 4 x 500va transformers for front end and output stages true to Richard's bigger philosophy.
I recall your build did not use centre tapped rails as per NVA. How did you create an ground input for the boards and speaker outputs ?
I only ask as I think I can do just -/+ rails for the output transisitors on my A60 board ie no ground is required for that part of the board ( using a spare -/+ rectifier/cap board ). I can use the centre tapped ground from the front end transformers to give me my earthing for the boards/output I think.

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Re: Dual mono or separate transformers for input- and outputstage?

Unread post by Ithilstone »

From my experience of building and repairing older amps and from endless talks with Richard the best solution is: 4 transformers, one for each input and output stage of each channel.

The next best ( as used in some older statement amps) is one transformer for both input stages and 2 separate transformers for output.

Next would be 2 transformers - one for input and another for output stages (as A60+) or one per channel (as A70). Different compromises that roughly balance each other.

Last would be one transformer to run it all (as A60).

BTW I also have seen AP70 from old days where Doc used 2 transformers - one per board but each supplying 2 bridge rectifiers with separate bank of smoothing caps - each BR supplying input and output separately ( on top of that output supply had some expensive bypasses fitted)

I think that at that time Richard was trying different things and in the end he always ended up with the simpler solution as the one he liked.

Regarding voltage and VA rating: yes in the past and only for biggest amp A80 and statement we used 25v for inputs and 30v or more for outputs also bigger VA for outputs than for inputs but from what Richard said it was more to do with limited space inside cabinet than anything else as he loved symmetry and where possible same values were used.

Currently we use only 25V, and with the A80 we use 150VA for both input and output.
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Hannes-Gregor (Tue Jun 30, 2020 1:54 pm) • CN211276 (Tue Jun 30, 2020 2:05 pm)
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Re: Dual mono or separate transformers for input- and outputstage?

Unread post by Geoff.R.G »

karatestu wrote:
Tue Jun 30, 2020 6:24 am
Obviously with such a low constant current draw, the input stage does not require as big (VA) transformer as the output stage. But then when you look at the lengths Richard would go to with transformer size (phono3) then maybe bigger is better for the input stage too. Bigger =better transformer regulation and lower impedance.
I don't think it is particularly obvious that a low power circuit doesn't need a very powerful transformer. The Phono3 has separate PSUs, 3, one for each gain stage and that means the PSU capacity overall is three time that of a Phono1. The next stage after the Phono is the Power Amp input stage and, although it doesn't draw much current it would be wrong to suggest that the current is constant, for a high input signal more current is required through the transistors to provide the required output voltage across the load resistor. Thus, although the current may not be large a high capacity PSU is required to prevent throttling (basically what is going on in the Phono 3). The faster the PSU can deliver the higher current the better. The same applies to the output stage.

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