Power Supply for Input Stage vs Output Stage

Hello Everyone,
I’m trying to rebuild the power supply for my old solid state power amp.
The amp board can be clearly broken down into the input stage and output stage, and I plan to feed each stage with dedicated power supply (rectifier, transformer, smoothing caps etc).
In the case of cost constrain, I can only afford to have a bigger transformer (say 250VA) and a smaller one (say 150VA), is it more correct to have the bigger power supply feeding the output stage and the smaller to the input stage?
Thanks
 

Vovk Z

Member
2011-10-30 10:32 pm
Kyiv
It is ok to have two power supplies for one amp, but it can be circuits where these power supplies must be synchronized (with using diodes or etc). I mean they may have to rise simultaneously (and/or drop).
I don't know what an amp it is, but usually "Pre"-stages consume only about 1-10 % of overall amp consumption. It is good idea to calc or to measure that consumption and investigate a need to sync those supplies.
 
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OP, please show a schematic of the amplifier. Then we may be able to determine whether your plan makes any sense.

I am not aware of any power amplifier using entirely separate power supplies like that, even though it would be doable with an affair using multiple differential pairs or similar. But as stated, the input parts of the amplifier usually take only a small fraction of the power. Consequently, amp designers tend not to bother. Sometimes they will be using separate taps on the same transformer though, such as when driving MOSFET output stages (which have several volts worth of Vgs, so a slightly higher supply for the VAS is beneficial).
 
It is rare to burn a transformer if an amp was fused. Mostly the electrolytic caps get dried up and cut power available to a watt or two, or start leaking and tripping the breaker.
Modern transformer suppliers, as the popular in the US antekinc.com , tend to have both a +- rail amp windings and +-15 op amp windings on all stock. So the system only needs one core, one primary winding, one circuit breaker.
Switcher supply vendor connex also sells +- rail voltage plus +- 15 op amp voltage in his offerings.
 
Hello Everyone,
I’m trying to rebuild the power supply for my old solid state power amp.
The amp board can be clearly broken down into the input stage and output stage, and I plan to feed each stage with dedicated power supply (rectifier, transformer, smoothing caps etc).
In the case of cost constrain, I can only afford to have a bigger transformer (say 250VA) and a smaller one (say 150VA), is it more correct to have the bigger power supply feeding the output stage and the smaller to the input stage?
Thanks
It is not ONLY more correct, it is necessary to have a bigger transformer for the output stage which is a current amplifier. The output stages dumps higher current onto the low impedance speaker loads. The input and driver stages are voltage amplifiers which need significant less current, thus, less power. Many power amplifier use higher rail voltage for the input and driver stage to avoid hard clipping.

If you are not planning on feeding the input stage a higher voltage, it is better to get a single 400 VA transformer which will give you more output current capacity to allow driving lower impedance loads if the output devices allow.

The Antek AS-4432 (400VA 32V x2 or 64VCT) Toroidal Transformer is a good fit for what you need. But the shipping could be expensive since you are not in the US.
 
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It’s actually an old NVA amp, I have an old A60.
As seen here?

Dreadful, just dreadful. Mega current loops with flying wires (see distortion #6), awful oldschool PCB layout with V+ and V- on opposing sides (would check for #5 and #7 as well), little in terms of bias thermal tracking (so it's all the better that bias seems to be chosen a little on the lean side to begin with), no capacitor agross bias diodes, monolithic Darlington outputs... There are DIYers who would be utterly ashamed of this kind of construction standard. At least parts quality seems to be good.

Realistically speaking, these abominations need at least rewiring to an actually decent construction standard, or should even be gutted and rebuilt altogether. Ideally by someone who has read the power amplifier books by Messrs. Self and/or Cordell.
 
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Thanks for the links Klass, lots to go through.
NVA is very old school and there are many strong critics about it's deployment and the workmanship.
Nevertheless I do enjoy the amp very much and I was deeply impressed by the A80 when I last heard, and I know it is basically using the same amp boards but separating power supply for the input and output stages.
Will go through and learn :) Many thanks