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Amp Camp Amp - ACA
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Old 30th September 2018, 01:32 PM   #6161
poseidonsvoice is offline poseidonsvoice  United States
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Amp Camp Amp - ACA
Semmyroundel,

Both Alan and Jim T are right.

But we don’t know enough about your system overall, particularly your source (is it 2V RMS or more?), your speaker’s overall efficiency, your listening position/distance from the speakers and your listening habits (ie how LOUD do you listen, in SPL; you can use a mobile phone app to measure this).

Even without knowing all of those details, my recommendation would be to have a preamp with a minimum gain of 6dB around. It will work great as not all recordings (from your source, assuming digital) is at 0dB fs (max volume). You will have some “quieter” recordings too that you need to compensate for.

Best,
Anand.
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Last edited by poseidonsvoice; 30th September 2018 at 01:55 PM.
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Old 30th September 2018, 01:43 PM   #6162
Alan4411 is offline Alan4411  Wales
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Quote:
Originally Posted by damkel View Post
The output from the phone fluctuates a bit but on average 0.37v.
Out ACA without speakers 1.4v.
Out ACA with speaker 1.17v.
This gives an amplification of 3.2 and not that far from your 3.6 and probably well within measurement uncertainty.

So I guess the conclusion is that my ACA is OK which is good news.

I connect the ACA to my desk speakers and used my phone as input. Really low output. Then I used the output on my pc (not laptop) and that changed everything. Suddenly I got a sound level that was usable.

So... how do I know if a preamp is usable?
How much gain is required?
None of my present gear works and I before buying (or building) a preamp I would like to be sure I am doing the right thing.

A nice sunday to all you ACA'ers :-)
Glad to know you are a bit better.
Yes your figure of 3.2 is spot on. It is working fine.

The pre-amp question is difficult to answer with a ''get this one'' recommendation. It will depend on your source in the first place and what voltage it gives out.

Both my CD player and Squeezebox have 2 volt outputs. Neither is enough to drive the ACA to near full volume.

But you only need a small amount of extra gain. 2 times(6dB) or 3.2 times(10dB) are fine. (Just make sure it is not a buffer or unity gain stage, it has to have gain.)

I have tried several types, Naim, NAD, valves etc, they all give enough to get to clipping and sound fine. At this moment I have a 'Death Of Zen' in there http://sound.whsites.net/project37.htm , I really like it with the ACA. (It was a junk box find too! Just needed a case and power TX.)
Next though I am collecting parts for a MooseFet to try that e0ltaipa spoke about. DIY Audio Projects Forum • MooseFET Preamplifier It is a very simple design just like the ACA. And it might be able to share the 24 volt power from the ACA brick...

Alan
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Last edited by Alan4411; 30th September 2018 at 01:51 PM. Reason: ooops picture missing.
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Old 30th September 2018, 02:00 PM   #6163
maxijazz is offline maxijazz
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Hi,
I have just built my first ACA 1.6. I do have MeanWell 24V power supply, but it outputs only 22.5V, when measuring with my DMM.
My concern is, what should be bias voltage? 12V or 50%x22.5V=11.25V?

Cheers,
Max
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Old 30th September 2018, 02:04 PM   #6164
Mooly is offline Mooly  United Kingdom
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Set your voltage to 50% of the measured supply voltage. Use the same range on your meter for both measurements.

Differences of a volt or so will make no practical difference to the perceived maximum output before clipping.
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Old 30th September 2018, 02:11 PM   #6165
maxijazz is offline maxijazz
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Thank you Mooly.

Another question...
I used to have class B and D amplifier and they used to be warm. I estimate their temperature used to be about 45-50 Celsius.
I have the CA powered up for about 30-40 minutes and it is really hot (it was getting hotter and hotter with every minute). It is hard for me to say, how much, but i think it is more like 70-80 Celsius now. Is that normal?

Cheers,
Max

Last edited by maxijazz; 30th September 2018 at 02:18 PM. Reason: clarified that it ACA is really hot, hard to touch longer than several seconds
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Old 30th September 2018, 02:19 PM   #6166
Mooly is offline Mooly  United Kingdom
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Absolutely normal for it to run really hot. That is one of the downsides of Class A

To be sure all is well you can measure the voltage across R1 and R2 (both 0.47 ohms). You should have approx. 0.375v across each which if you calculate the current using ohms law will be around 0.8 amps per resistor and 1.6 A total current draw.

Be careful not to short anything out if you check this.
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Old 30th September 2018, 02:30 PM   #6167
maxijazz is offline maxijazz
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Thank you very much Mooly!

May the force be with you!
I measured 0.307V in left channel and then 0.300 in right one.

Cheers,
Max
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Old 30th September 2018, 02:38 PM   #6168
Mooly is offline Mooly  United Kingdom
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That sounds fine and confirms the amp is not drawing to much current.

The heat is normal and the product of the current multiplied by the voltage... so in total its 1.27 amps * your 22.5 volt supply which is 29 watts dissipation per channel.
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Old 30th September 2018, 04:07 PM   #6169
semmyroundel is offline semmyroundel  United Kingdom
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Quote:
Originally Posted by Alan4411 View Post
The answer is yes or no*.

Mono 'bridged' or 'balanced' mode (2 ACAs) gets more power out for 8 to 16 ohm impedance speakers so say 14 - 15 watts. They can effectively swing more volts.
'Parallel' mode (again 2 ACAs) gets more power into lower impedance speakers. 4 ohms or so. Could be 15 watts. This is because there is double the current available.

* So the answer depends on the speaker impedance as much as anything.

There are puts and takes for either mode and it is covered here https://cdn.shopify.com/s/files/1/10...ation_RevA.pdf in the build guide.
Thanks Alan, but they are still mono apart from the basic 8watts stereo version.
I haven't measured them exactly, but I assume that all inputs are line level (around 2v), my speakers are 8 ohms.
The target speakers are either Qacoustics concept 20's or Mordaunt short pageant series 2 (which I'm redesigning the cabinets for).
The Pageants are around 93db/95db, and the concept 20's around 85db, BUT I didn't want to be tied to any particular speaker, though for the purposes of these questions, I understand I need to be at least for now

Last edited by semmyroundel; 30th September 2018 at 04:10 PM.
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Old 30th September 2018, 06:02 PM   #6170
Toaster is offline Toaster  United Kingdom
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Pageants aren't going to be that sensitive. Somewhere around 88-90dB from memory.
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