View Full Version : Identical amps, different output level?


Saigo
06-04-2010, 10:54 PM
I have a pair of amplifiers (RCA Dimensia MPA120, if it matters) that I am using to bi-amplify my Carver AL-III's. The amplifiers are identical, but one puts out significantly higher volume than the other at the same input level from the preamp. I've ruled out everything upstream, so the problem is definitely in one of the amps. As far as I can tell, the gain of these amps is entirely non-adjustable; they have no external adjustment, and I can find no pots of any kind anywhere on the boards.

Can anybody offer any suggestions to explain and correct this behavior? The only way I can address the situation at present is to run two preamps. This is obviously irritating as hell, and I'm determined to fix it.

Thanks.

Oerets
06-04-2010, 10:56 PM
Have you switched the inputs to the amps to see if the problem moves ?



Barney

Saigo
06-04-2010, 11:07 PM
Have you switched the inputs to the amps to see if the problem moves ?

I have. I am feeding both amplifiers from a B&K Pro 10 MC preamp via a single RCA cable with two Y adapters at the end. I switched the Y adapters around with no effect. I also ran the RCA directly to one amp at a time, playing the same passage of music at the same preamp volume level, and one amp definitely has a higher output level than the other. By the meters, when the louder amp is putting out 1 watt, the quieter one is putting out about .1 watt.

Oerets
06-04-2010, 11:55 PM
Sorry , zero experience with you setup . Looked abit on the net , still know squat . Can't find any info so far on those amps .

Do they have any filters or switches that could need cleaning ? Are they Mono or are you running them in Bridge mode ?

When you say Bi-Amping how do you have it hooked up ?

Also did it work fine but now is funky ?


Barney

Saigo
06-05-2010, 12:08 AM
I appreciate all the research you're doing. It's definitely above and beyond anything I expected.

Concerning the amps, you aren't going to find much info. These are oddballs, available only (as far as I know) as part of the RCA Dimensia audio video system. It was high end for RCA, whatever that means. But they're really pretty basic two channel amps, putting out either 100 or 120 WPC (claims vary). They have no bridge mode. So when bi-amplifying the Carvers, I'm using one amp to drive both ribbons, and the other amp to drive both woofers.

I hadn't thought about the switches, although I should have. I possess De-Oxy-It, and I should put it to use.

And this setup has always been this way. Truth to tell, I only received the second amp today, and I have been kicking this bi-amplified setup around every which way trying to balance it out.

bowtie427ss
06-05-2010, 12:15 AM
have you tried switching the amps around, meaning put the one currently driving the ribbons on the woofers and vice versa?

I'm wondering if you're seeing/hearing how the amplifiers react to different loads.:scratch2:

Oerets
06-05-2010, 12:18 AM
When Bi-amping you don't need a alot of power for the highs , well not near as much as the low end will .


Barney

Saigo
06-14-2010, 10:47 PM
I have tried switching the amps, and one is definitely putting out more volume than the other--whichever input on the speakers it is connected to, whether high or low, gets louder. I've made no progress with this, so short of tearing into both amplifiers and comparing things, I'm stuck with the two-preamp solution.

It is curious, though, that the protection circuit in the quieter amp behaves strangely: It trips off immediately after the amp comes on, whereas on the other, louder amp, the protection circuit stays active for the customary few seconds after startup. Since this data point seems unusual, perhaps the quieter amp is the one I should be looking at.

laatsch55
06-14-2010, 10:55 PM
I would look to the quieter amp first. Have you done any preliminary measurements such as DC offset and bias?
I've had some amps exhibit this and traced it to a blown B+ fuse or Pre-drives, or Driver transistors. Could be power supply problems too. A good thing in your favor is you have another one for comparison.

whoaru99
06-14-2010, 11:38 PM
When Bi-amping you don't need a alot of power for the highs , well not near as much as the low end will .


Barney


That's not necessarily true.

If you're feeding the biamp to the highs through the original crossover just with the jumpers removed, you have to feed the same level of input signal otherwise the highs will not be at the correct level to match with the lows.

Still Crazy
06-22-2010, 01:46 PM
Saigo,

I have not posted before, choosing to remain silent rather than reveal my lack of knowledge. In this case I am compelled to join the conversation for I am also working with an MPA 120 with curiously low gain compared to my other amps. I do not have multiple 120's, but I did notice how low the gain was on my 120 compared to other components - which I thought was odd. My low gain 120 fires up with the protection light on for a couple of seconds and then clicks off. I confirmed that the +/_ rails are at 66 volts so it doesn't look like anything related to that. Let me know if you find a fix.

Saigo
06-22-2010, 02:21 PM
In this case I am compelled to join the conversation for I am also working with an MPA 120 with curiously low gain compared to my other amps.

No shit? Somebody else out there actually has one of these things? I'm relieved, because while I've had some good general replies, I was beginning to think I was on my own with anything specific to this design.

I confirmed that the +/_ rails are at 66 volts so it doesn't look like anything related to that. Let me know if you find a fix.

I haven't checked the rail voltage on mine, because while I have a bit of experience with home and auto electrical, I am still very new to hi-fi electrical. If you can tell me the procedure you used to check yours, I'll check the rail voltage on both of mine and report back.

Also, I can't see anything that would allow me to check or adjust the DC bias, which I have just done on my Nikko Alpha II via pots on the main amplifier boards. Do you have any ideas on how to check that in these amps?

Thanks.

Saigo
06-22-2010, 02:24 PM
I would look to the quieter amp first. Have you done any preliminary measurements such as DC offset and bias?

I haven't measured either of these because so far I don't know how to do so on these amps.

I've had some amps exhibit this and traced it to a blown B+ fuse or Pre-drives, or Driver transistors. Could be power supply problems too.

This is beginning to sound like I'll need a schematic. Considering how little known these amps are, I am uneasy about the prospect of finding one.

Still Crazy
06-22-2010, 03:28 PM
The +/_ rails are marked on the PC board (-VCC and +VCC) in front of each of the Power Packs. Neg meter lead to the case for ground and then measure the wire jumpers near the identifiers. Kind of like playing operation, be carfull not to short anything out! Best not to bother if you don't like such activities. IMHO if the amp is producing sound the rails are good to go since the protection circuit usiually won't kick in if the voltages are wrong.

Mine is running the STK-4046XI, there is another version called the 4046V but all the info I find on these packs online is the same. As far as adjustments I don't see any - my experience with the STK packs is that they are internally balanced and don't allow adjustment. Mine are dead on with only a few mv at the outputs. As for the gain I suspect that it is possible that RCA may have made a hardware change somewhere along the line to align the gain with other components in their family of products, any date code identifiers which would give a clue?

Saigo
06-22-2010, 03:36 PM
I'll check when I get back to the house this evening. I've been meaning to swap the Onkyo M-506RS back in anyway, at least until I can sort out the difference between these two RCA amps.

jwalker
06-22-2010, 04:01 PM
That's not necessarily true.

If you're feeding the biamp to the highs through the original crossover just with the jumpers removed, you have to feed the same level of input signal otherwise the highs will not be at the correct level to match with the lows.

You are confusing gain with power.

It's true that the gain must be at the "correct level" and the same for a passive biamp application. However in practical use with music, more power is usually needed at lower frequencies. It's important to realize that music is NOT white noise.

dspear99ca
06-22-2010, 05:30 PM
Many power amps have a gain control for each channel so you can balance two discrete amps. Although I'm sure you could thoroughly analyze your amps, comparing voltages at varying points in the circuit between one vs. the other, wouldn't it be a whole lot simpler to just put an attenuating trimpot inline between the pre and power amps and match the levels manually? I had a similar issue with an H/K integrated a few years ago, spent quite a lot of time trying to figure out why one channel was louder than the other. It was suggested that I just adjust the balance knob to compensate, which I did and which is still working perfectly...

pradigmshift
06-22-2010, 07:42 PM
I would look to the quieter amp first. Have you done any preliminary measurements such as DC offset and bias?
I've had some amps exhibit this and traced it to a blown B+ fuse or Pre-drives, or Driver transistors. Could be power supply problems too. A good thing in your favor is you have another one for comparison.
+1
It is curious, though, that the protection circuit in the quieter amp behaves strangely: It trips off immediately after the amp comes on, whereas on the other, louder amp, the protection circuit stays active for the customary few seconds after startup. Since this data point seems unusual, perhaps the quieter amp is the one I should be looking at.
Indeed. With a meter start measuring voltages, esp. the mains + and - and driver/power stages. If you can get a manual compare with it otherwise compare the two units.
wouldn't it be a whole lot simpler to just put an attenuating trimpot inline between the pre and power amps and match the levels manually?
Yeah, but its more comforting and safer to fix the problem. For example it might be that currently only the + side of the signal gets amplified...

Just my 2 cents..

laatsch55
06-22-2010, 11:14 PM
pradigmshift, exactly the symptom of say, a PL 400 when the +/- rail fuse blows but not the other, reduced , noisy output.

sregor
06-23-2010, 02:07 PM
My 2cents. Only time I encountered this problem was when the electrolytic going to ground in the feedback circuit was open. Made the channel unity gain instead of the usual 26 to 30 db. Measure input and output voltages. Good luck with it.

Still Crazy
06-30-2010, 11:09 AM
Victory!!!

sregor gets the gold star on this one. Since both channels of my amp were doing the same thing I decided it could not be two failed caps, so I went after the gain resistors. After some hunting around on the PCB I was able to find what I was certain was the right part, but changes in resistor values had no effect. Kind of like cranking the volume knob with no change in the sound output! So, I replaced the cap on one of the feedback circuits and there was a solid gain in the output of that channel. Further resistor swapping on the updated channel verified that gain could now be modified by changing the values. Looks like the caps were the weak link on my amp and could be on other MPA120s as well.

Thanks for the insight sregor.

Saigo
06-30-2010, 11:29 AM
That's a fine thing, and the kind of thing that with my limited experience of electronic diagnostics I probably would never have found. Can you post a description or photo of the location of the caps that you replaced and their values?

Thanks.

Still Crazy
06-30-2010, 02:02 PM
Capacitor value is 47 microfarads rated at 6.3volts. I didn't have a 6.3 volt part so I used a higher voltage rating so it is slightly larger in size.

JPG 1 is the left channel. The cap that was replaced is slightly larger than it's neighbor - the PCB label is C703L. JPG 3 is the right channel. The cap that was replaced is right next to the two big guys, it has part number PCB C703R.

Sorry for the low quality phone pics, it was all I had.

BinaryMike
06-30-2010, 03:46 PM
Since both channels of my amp were doing the same thing I decided it could not be two failed caps, so I went after the gain resistors. After some hunting around on the PCB I was able to find what I was certain was the right part, but changes in resistor values had no effect. Kind of like cranking the volume knob with no change in the sound output! So, I replaced the cap on one of the feedback circuits and there was a solid gain in the output of that channel. Further resistor swapping on the updated channel verified that gain could now be modified by changing the values. Looks like the caps were the weak link on my amp and could be on other MPA120s as well.

One of the 'secrets' known to proficient technicians is relative failure rates for different types of components. A good tech would go after the resistors only after verifying everything else good. It won't pay off in every case, but it's an important lesson that newbies can benefit from.

Still Crazy
07-07-2010, 11:01 PM
BinaryMike, your feedback is appreciated. There is alot of fun to be found in this old gear! Sometimes the quest to get things running is half the fun.

The MPA is up and running fine, had to do a repair on the protection circuit as well.

Such Fun!

Saigo
07-08-2010, 11:41 AM
Would you mind giving me a quick description of the repair on the protection circuit? I think that I may be up against the same in my MPA120, since the protection circuit clicks off immediately when the amp comes on, and this isn't the correct behavior.

Thanks.

Still Crazy
07-09-2010, 02:40 PM
In my case I had a bad 220uF cap. next to the 8-pin inline ( connection was to the 2nd pin if I remember correctly) protection chip near the front display panel. The circuit was kicking in whenever a music peak hit about 60 watts or so on the display.

Does your protection circuit go to normal operation immediately? If so does the amp function OK? It may be that the delay function of the protection chip is not working properly.

It seems that the capacitors on this run of amps are prone to failure, not that 25 years is a short life span for an amp.

benb
07-09-2010, 05:38 PM
Victory!!!

sregor gets the gold star on this one. Since both channels of my amp were doing the same thing I decided it could not be two failed caps, so I went after the gain resistors. After some hunting around on the PCB I was able to find what I was certain was the right part, but changes in resistor values had no effect. Kind of like cranking the volume knob with no change in the sound output! So, I replaced the cap on one of the feedback circuits and there was a solid gain in the output of that channel. Further resistor swapping on the updated channel verified that gain could now be modified by changing the values. Looks like the caps were the weak link on my amp and could be on other MPA120s as well.

Thanks for the insight sregor.

Capacitor value is 47 microfarads rated at 6.3volts. I didn't have a 6.3 volt part so I used a higher voltage rating so it is slightly larger in size.

JPG 1 is the left channel. The cap that was replaced is slightly larger than it's neighbor - the PCB label is C703L. JPG 3 is the right channel. The cap that was replaced is right next to the two big guys, it has part number PCB C703R.

Sorry for the low quality phone pics, it was all I had.
As far as components that go bad, especially over a long time period, electrolytic capacitors are often suspect, ESPECIALLY ones with substantially lower voltage on them than their rating, such as in this feedback/gain setting area. Just about every solid state amp has this. 6.3V is about as low a rating as they make for electrolytics, and the actual voltage across them in normal operation is surely always well below 1V. It's about the same as the output offset voltage

A higher voltage rating is fine for a short term replacement, but you'll have to remember in 10 or 20 years to replace it again.
In my case I had a bad 220uF cap. next to the 8-pin inline ( connection was to the 2nd pin if I remember correctly) protection chip near the front display panel. The circuit was kicking in whenever a music peak hit about 60 watts or so on the display.

Does your protection circuit go to normal operation immediately? If so does the amp function OK? It may be that the delay function of the protection chip is not working properly.

It seems that the capacitors on this run of amps are prone to failure, not that 25 years is a short life span for an amp.
I suspect this cap also doesn't have much voltage on it during normal operation. It's as much a basic electrolytic capacitor problem as much as anything. There's a way to design around a cap having no voltage on it, but it takes more components.