ALchemist adp20a power amp. Low output - diyAudio
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Old 12th April 2015, 03:59 PM   #1
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Default ALchemist adp20a power amp. Low output

Hi, This is my first post. I have an Alchemist Adp20a power amp and adp21a pre amp which have not been used for many years. On connecting them up recently I notice that the perceived output volume was much lower than I expected. Sound was very clean and warm as I remembered, just the volume was low. Is it likely that the electrolytic capacitors in the power amp need replacing. I look forward to your thoughts
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Old 12th April 2015, 04:59 PM   #2
Mooly is offline Mooly  United Kingdom
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Hmmm

Are you sure it is really 'low in output' or is it perhaps 'the position of the volume control is higher than I expected' ?

It is very very rare to get a true fault causing any gain variation away from the designed value. Stereo amp ? both channels the same ? If so then I truly doubt there is an issue with the power amp.

This kind of thing can be proved definitively if you have a circuit diagram (the gain can then be calculated) and a scope or even a DVM (to measure the output for a known applied input voltage)
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Old 13th April 2015, 12:36 PM   #3
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Sorry about the delay in replying. I shouldn't let business get in the way of pleasure.

The volume control is way higher than it used to be. At Max volume I was getting a sound level which was not filling a small room. for testing I was using a pair of monitor audio bronze b1's which at rated for about 60w @8ohms. The Alchemist adp20 s outputing 150w/CH into 8ohms. I checks with a 30w power amp and should get the found to uncomfortable levels.

I have been unable to locate a schematic for his amp. I purchased it in the 1990's. Manufacturer has since gone out of business.
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Old 13th April 2015, 05:37 PM   #4
Mooly is offline Mooly  United Kingdom
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This kind of issue has to be confirmed by measurement. You see the gain of an amplifier is determined by what is called the 'feedback factor' and that in most cases is set by just a couple of components. Also if both channels are behaving the same then that is pretty much a definite bit of evidence that whatever you are experiencing isn't directly caused by the power amp.

Are you using the same preamp as you used to use ? Could the problem be in that area ? All I could find was the user manual which states '1 volt input sensitivity'. That's a pretty average figure but I did notice the stated input impedance was low. That would/or could make things preamp dependent.

So if you are sure in your own mind the problem is with the power amp then you have to prove it measurement... which is actually pretty easy and two minutes of a job.

Ideally you need a scope and signal generator but a good DVM and test disc will suffice. You need to make a test disc (CDR or CDRW) of a sine wave tone of high amplitude and of around 400Hz. You play the disc and starting with the volume on minimum measure the voltage at the power amp input and across the power amp output. That will give you gain. 150 watts rms equals 35 volts rms. So if the sensitivity is 1 volt then the amp should have a numerical voltage gain of 35. So if you apply 20 millivolts for example then you should see 0.7 volts AC across the speaker. So in practice you just turn it up a little and then measure the two values and calculate.

You can make a test disc easily in Audacity which is free.
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Old 13th April 2015, 07:35 PM   #5
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Thanks. Problem is with the power amp.I have tried the pre amp with another power amp(of much lower rating) and achieved much higher output.

I'll try the scope and a test disc and hope the scope is in better condition than the amp despite it being in storage for much longer
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Old 14th April 2015, 06:24 AM   #6
Mooly is offline Mooly  United Kingdom
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Great ! You have a scope

Remember to measure the input voltage with it coupled up and working in order to get a true result (you can run without speakers to get a higher output level without it being painful to listen to). The user manual for the amp showed 25k ? as an unbalanced input impedance and 8k ? for balanced.

Any switches or jumpers set incorrectly on the power amp ?
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Old 14th April 2015, 07:55 AM   #7
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to measure 20mVac, or 50mVac, you need a DMM with a scale that goes to 199.9mVac.
using a 1.999Vac scale loses accuracy.
using a 19.99Vac scale is a waste of time.

Test/measure the input and output voltages with no speakers connected, then with one speaker connected then with both speakers connected.
You may find very small changes in the output voltages.
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Old 14th April 2015, 10:10 AM   #8
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I would start hunting for contact issues first.

See whether cranking the volume improves things. If so --> clean/replace protection relay.
I see socketed DIP components inside (opamps and stuff). With the power off, reseat those (if you don't have a chip puller, gently lever with a flat-head screwdriver or two from both sides).
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Old 14th April 2015, 03:00 PM   #9
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there is got to be something else ..neither capacitors nor contacts behave like that so let us expect the OP to give us some other hint

Kind regards
sakis
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