I don't think so. It definitely should not be so anyway if the values used are the same in the two amplifiers 🙂
The gain really is set by the component values around the feedback network and one of the great benefits of negative feedback is that is removes this variability that could occur due to big differences in things such as the active parts (the transistors).
I think at this point you have to do a real meaningful test and get a figure for what sort of difference there is which is easy to do.
All you need is a lowish frequency test tone such as a 400 Hz sine wave test tone. I'm assuming, maybe wrongly, that you haven't a scope and will be using a DVM on its AC voltage range for this. Obviously if you have a scope...
So without speakers connected we play the tone and set the volume to give say about 5 volts AC across the speaker terminals. Without touching the volume control now measure the AC voltage going in to the amp on R11. Measure on the resistor itself and measure at the end of the resistor that goes to the input sockets. Be sure to measure on the resistor itself though.
Divide the measured output voltage by the measured input voltage to get the gain.
When testing the second amp do not alter the volume setting of the preamp. If there is any difference in the input configuration between the amps this might highlight that.
Write down the measured output voltage of the second amp. It should be the same as above (5 volts AC)
Now measure the input voltage on R11. Is it the same as for the first amp? If not the same then write down the result.
Calculate the gain for this second amp in the same way by dividing output voltage by input voltage.
(there is one possible variable in measuring this way and that is the meter. If your preamp has a high output impedance then the meter might load the input signal effectively reducing it... but at this point the test is worth doing)
@ ryan391.
We assume you have tested the 'quieter' new ACA in normal stereo mode and you do get equal outputs from both halves of the amp? (Not unkown for one side not to be working on stereo, but does give low output when Bridged.) But that will become self evident once you do Mooly's test.
Also the rear switch should be in the middle position when using XLR Bridged, Build guide step 15 https://guides.diyaudio.com/Guide/Amp+Camp+Amp+V1.8+Change+Information/10?lang=en#s348
We assume you have tested the 'quieter' new ACA in normal stereo mode and you do get equal outputs from both halves of the amp? (Not unkown for one side not to be working on stereo, but does give low output when Bridged.) But that will become self evident once you do Mooly's test.
Also the rear switch should be in the middle position when using XLR Bridged, Build guide step 15 https://guides.diyaudio.com/Guide/Amp+Camp+Amp+V1.8+Change+Information/10?lang=en#s348
This is a lot more advanced than I have had experience with. I do not have a scope. If I do all this and the values are off, what is the fix? Also, I had a different pre amp to start and have since changed. The result was the same on both pre amps. So pre amp is not the problem.
Well that is what we need to determine 🙂 We can't say what the fix is until we can define the problem exactly.
We have to start somewhere and basic tests and measurement are key to that. The theory is sound, the gain of the ACA (and any other amp) is determined by the feedback factor... so we need a handle on it to see what is happening.
If you need a test tone I can post an MP3 file that will play on any suitable player/phone/PC etc that can be used a a test source.
Darn.
1. could you possibly facetime me through this?
2. MicroCenter (an electronics store) has a very generous return policy. I could purchase a scope and return it after I use it. Not the most ethical thing to do, but if that makes it easier.
1. could you possibly facetime me through this?
2. MicroCenter (an electronics store) has a very generous return policy. I could purchase a scope and return it after I use it. Not the most ethical thing to do, but if that makes it easier.
Are you sure the resistor those resistor values l
Ooh I don't do any of that stuff 😀 but I can give all the help needed via the forum 🙂
One thought... I have it in my head from somewhere (and can't find anything printed about this) that some ACA's from way back used a 68k instead of a 39k for R12.
Where have I got that from? Can anyone confirm that?
Have you checked in the other amp that the values used for those resistors are the same. A 68k would double the gain.
Although a scope is the best tool for the job a quick meter check with a test tone is worth trying. All you have to do is play the tone back and take two voltage readings. You would still need the tone using a scope.
Ooh I don't do any of that stuff 😀 but I can give all the help needed via the forum 🙂
One thought... I have it in my head from somewhere (and can't find anything printed about this) that some ACA's from way back used a 68k instead of a 39k for R12.
Where have I got that from? Can anyone confirm that?
Have you checked in the other amp that the values used for those resistors are the same. A 68k would double the gain.
Although a scope is the best tool for the job a quick meter check with a test tone is worth trying. All you have to do is play the tone back and take two voltage readings. You would still need the tone using a scope.
1. I do need a test tone if you can post that.
2. These amps were purchased together. I had problem with the quieter one getting both channels to play. I rebuilt it and now plays but at a lower volume
I really enjoyed building these, but I hate the problems. I want to just be done already 🙂
2. These amps were purchased together. I had problem with the quieter one getting both channels to play. I rebuilt it and now plays but at a lower volume
I really enjoyed building these, but I hate the problems. I want to just be done already 🙂
I'll do a tone now... give me a couple of minutes.
It is worth you visually checking that the same value parts for R11 and R12 are fitted to both amplifiers. Don't assume anything 🙂
This is what you are checking. This shows voltage in and voltage out as a scope would show it and (in the boxes arrowed) what a meter will show.
The gain works out as the 39.2k divided by the 10k (so numerically 3.92) and you can see this simulation comes pretty close at around 3.7. Why is not exact... because of the complication of R10 and the setting of the preset which is an unknown as each amp is different.
Give me a minute for the tone...
It is worth you visually checking that the same value parts for R11 and R12 are fitted to both amplifiers. Don't assume anything 🙂
This is what you are checking. This shows voltage in and voltage out as a scope would show it and (in the boxes arrowed) what a meter will show.
The gain works out as the 39.2k divided by the 10k (so numerically 3.92) and you can see this simulation comes pretty close at around 3.7. Why is not exact... because of the complication of R10 and the setting of the preset which is an unknown as each amp is different.
Give me a minute for the tone...
Here you go. Zipped file to attach to the forum.
The tone plays for 60 seconds, is in stereo (2 channel) and has 5 second fade in and out. MP3 @96kbps
The tone plays for 60 seconds, is in stereo (2 channel) and has 5 second fade in and out. MP3 @96kbps
I'll look in later 🙂
Remember to test without speakers connected (it would be to loud with).
Set the output voltage at the speaker terminals with the volume control (and its AC voltage remember) to be around 5 volts or so when playing the tone. The higher the better but it has to be below the amplifiers clip point. 5 volts is a good area to aim for but the absolute level does not really matter.
Now compare output voltage and input voltage as measured on R11 (the end that goes to the input sockets). Try and use the same range on the meter for both readings.
And that's it 🙂
Lets see what the difference between the two amps really is.
Remember to test without speakers connected (it would be to loud with).
Set the output voltage at the speaker terminals with the volume control (and its AC voltage remember) to be around 5 volts or so when playing the tone. The higher the better but it has to be below the amplifiers clip point. 5 volts is a good area to aim for but the absolute level does not really matter.
Now compare output voltage and input voltage as measured on R11 (the end that goes to the input sockets). Try and use the same range on the meter for both readings.
And that's it 🙂
Lets see what the difference between the two amps really is.
@ ryan391,
We are trying to help you, but you do not answer all the questions we ask. We need to get information we can make a sensible judgement on.
There is another way to use a test tone if you have a mobile phone. From here https://www.diyaudio.com/community/threads/amp-camp-amp-aca.215392/page-298#post-5547999
Quote, ''Here is a way to test your ACA is amplifying correctly. I just tried it on mine and this is what I did.
Used my mobile phone and went to this site for the 1 kilo Hertz test tone, it is a bit louder than some.
I turned my phone's volume to maximum and measured the AC voltage at the head phone socket playing the tone. Mine is 0.43 volts AC. Yours might be more or less, just note what it is.
Then I plugged the phone into my ACA and measured the output at the speaker terminals. (No speakers connected.) I get 1.622 volts AC.
If I connect my speaker (8 ohms) and measure I get 1.57 volts AC.
Again yours might be different just note it.
So on my amplifier for an input of 0.43v ac I get 1.57v ac out.
3.6 times amplification and you should get something similar if all is well.
Hope that helps.''
We are trying to help you, but you do not answer all the questions we ask. We need to get information we can make a sensible judgement on.
There is another way to use a test tone if you have a mobile phone. From here https://www.diyaudio.com/community/threads/amp-camp-amp-aca.215392/page-298#post-5547999
Quote, ''Here is a way to test your ACA is amplifying correctly. I just tried it on mine and this is what I did.
Used my mobile phone and went to this site for the 1 kilo Hertz test tone, it is a bit louder than some.
I turned my phone's volume to maximum and measured the AC voltage at the head phone socket playing the tone. Mine is 0.43 volts AC. Yours might be more or less, just note what it is.
Then I plugged the phone into my ACA and measured the output at the speaker terminals. (No speakers connected.) I get 1.622 volts AC.
If I connect my speaker (8 ohms) and measure I get 1.57 volts AC.
Again yours might be different just note it.
So on my amplifier for an input of 0.43v ac I get 1.57v ac out.
3.6 times amplification and you should get something similar if all is well.
Hope that helps.''
Attachments
Thank you, this was very easy to follow. I have discovered that the output is 1/2 of what the other amp is. Telling me one of the channels is not outputting. I think
The left channel with RCA is not responding, but right is. something is not correct. Both channels do register when i bias them though.
Both channels of an ACA should output the same voltage. There should be no interaction in normal stereo mode.
If one channel is 'not responding' (do you mean no output at all?) then the first thing to check is that you have not accidently crossed the output connections over. Remember that the ACA has the amplifier output going to the negative terminal while the positive terminal is actually ground.
If that is OK then you need to do the simple test of checking that the input signal is present on R11. Compare the reading with the good channel.
When i am in bridged mono it puts out Voltage out of two outside poles. When I change to stereo and plug into white. Nothing on white side poles. Plug into red. same voltage as bridged mono on red side poles
What is the best way to check voltage to R11?
With a meter if you have no scope. The meter must be on AC volts. Measure between these two points in red for the input voltage and then between the two points in blue for the output voltage.
Set the amplifier to normal stereo with the switch on the back.
Write down and post the results.
Set the amplifier to normal stereo with the switch on the back.
Write down and post the results.