I am working on a Carver TFM-6 amplifier that exhibits DC offset pumping. With every bass note, there is a large DC offset on the output. this causes the woofers of my test speakers to bottom out on every Bump of a bass note. this starts at a low level..maybe 1 watt and gets worse as the volume goes up.
When I got the amp it would not come out of protect as it had a bad capacitor in the Protection circuit. I suspect all the capacitors will need to be replaced. I have ordered a few of the values I don't have on hand but while I wait. I was looking at the schematic...nothing special class AB amp. and thinking that maybe the capacitor in the feedback path that provides the AC coupling to ground is the fault?
The other odd thing I noticed is that the Diff pairs of the L and R channels are both feed off the SAME Current source? which seems odd and bad! is it worth duplicating a 2nd source and tacking it on?
When I got the amp it would not come out of protect as it had a bad capacitor in the Protection circuit. I suspect all the capacitors will need to be replaced. I have ordered a few of the values I don't have on hand but while I wait. I was looking at the schematic...nothing special class AB amp. and thinking that maybe the capacitor in the feedback path that provides the AC coupling to ground is the fault?
The other odd thing I noticed is that the Diff pairs of the L and R channels are both feed off the SAME Current source? which seems odd and bad! is it worth duplicating a 2nd source and tacking it on?
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If changing the caps doesn't solve the problem, you may want to check for dry joints.
Regards
Mike
Regards
Mike
recapping this POS didn't help! But I did some poking around and noticed there is no separation between front end and driver stage... Normally I see a resistor and a couple of caps to stiffen the front end supply at least...So I tried that. Added some 22 ohm resistors in series with some 47uf caps to ground after the resistors. That helped. But didn’t cure the problem. I can turn it up louder now before the swinging DC gets bad.
I recapped the whole amp except main PSU caps. I have to find some that will fit the narrow 1ru chassis height. But ripple is not bad so I don't expect the caps to be that out of shape.
Maybe if I play around with the resistor values for the front end and up the size of the caps it will help a bit more but...what in the world is up with this design?? there has to be a fix for this thing?
ZC
I recapped the whole amp except main PSU caps. I have to find some that will fit the narrow 1ru chassis height. But ripple is not bad so I don't expect the caps to be that out of shape.
Maybe if I play around with the resistor values for the front end and up the size of the caps it will help a bit more but...what in the world is up with this design?? there has to be a fix for this thing?
ZC
At idle, the offset is about 0.04v so it's not a static thing, just every bump of a bass note causes a large negative 1hz swing.
I KNEW I had seen this schematic before! It's a ******* Parasound HCA-800II amplifier!! this is exactly the same amp!
Thanks for the schematic. I wasn't aware that they are the same amp. I'm sure I have a HCA-800II somewhere waiting for my attention.
I don't recall having the same kind of problem with any amps I've worked on. From what you described, it could well be a transistor issue. Could be the drivers.
I don't recall having the same kind of problem with any amps I've worked on. From what you described, it could well be a transistor issue. Could be the drivers.
even the carver PCB has the holes for the different input DC blocking caps as shown on the Parasound layout! I just kept looking at the schematic thinking that I had seen it before. But it was driving me nut's where...then I remembered the Parasound has the same goofy Internal Heatsink layout! and sure enough....same amp!!
Both channels exhibit the same problem. and it does it if you only run one channel.
I am going to try a different preamp today. I did not see any DC on the input. and the TFM6 has new input caps. which BTW are HUGE! 47uf input caps?? I am using to seeing 10uf input caps and I see the Parasound was a 4.7 and something smaller like maybe 1-2uf cap and a 0.1uf cap all in parallel for input caps. I put in Nichicon Muse 10uf input caps which still give something like a 2-4hz cutoff
But maybe the Carver CT6 I am using has an issue? They both probably should be recapped.
Both channels exhibit the same problem. and it does it if you only run one channel.
I am going to try a different preamp today. I did not see any DC on the input. and the TFM6 has new input caps. which BTW are HUGE! 47uf input caps?? I am using to seeing 10uf input caps and I see the Parasound was a 4.7 and something smaller like maybe 1-2uf cap and a 0.1uf cap all in parallel for input caps. I put in Nichicon Muse 10uf input caps which still give something like a 2-4hz cutoff
But maybe the Carver CT6 I am using has an issue? They both probably should be recapped.
Problem solved. It was the Carver CT6 preamp after all!!!
I was chasing the result and not looking at the real problem! a TC9215P in the output of the CT6 preamp had some DC offset on the input that would cause the positive half of the waveform to clip, when it would clip it would cause the DC to shift negative. this combined with the HUGE Input coupling cap in the TFM6 allowed what equated to a subsonic note into the amp and hence woofer bottoming on every big peak.
Tested with a different preamp, The amp works great and no odd issues. It had just been the timing of it that made my suspect the amp. when it arrived there was a bad cap in the protection circuit that kept it in protect mode. replacing that cap allowed the amp to work but I suspected all the rest of the caps to be bad as well. so I had been focusing on the amp being the problem. after I stepped away from it for a day i had the thought that the preamp was new too and maybe that was suspect...yup...it is....Parts ordered...
But...this also leads me to believe that some mods could be done to this amp to make it better...but why polish the proverbial turd? it works and will do what it needs to do well enough for my application.
I was chasing the result and not looking at the real problem! a TC9215P in the output of the CT6 preamp had some DC offset on the input that would cause the positive half of the waveform to clip, when it would clip it would cause the DC to shift negative. this combined with the HUGE Input coupling cap in the TFM6 allowed what equated to a subsonic note into the amp and hence woofer bottoming on every big peak.
Tested with a different preamp, The amp works great and no odd issues. It had just been the timing of it that made my suspect the amp. when it arrived there was a bad cap in the protection circuit that kept it in protect mode. replacing that cap allowed the amp to work but I suspected all the rest of the caps to be bad as well. so I had been focusing on the amp being the problem. after I stepped away from it for a day i had the thought that the preamp was new too and maybe that was suspect...yup...it is....Parts ordered...
But...this also leads me to believe that some mods could be done to this amp to make it better...but why polish the proverbial turd? it works and will do what it needs to do well enough for my application.
Glad to hear you've got it sorted out. I had the TFM6 and TFM15 in the 90s. Never used them much and yet after a few years, the TFM15 had the same protection issue. Output from the amp was fine though.
Regarding that Current Source, I think it's a voltage regulator. The resistor from the differential to the rails determines the standing current.
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