Slight sound degradation

adarrncsu

New Member
My office system uses the following: Preamp = NAD 1020, Amp = Parasound HCA-750A, Speakers: Polk Monitor Series 5B with Solen caps and Mills resistors, DAC = Maverick TubeMagic D2. In the last couple of days I've noticed a slight drop-off with respect to high end frequencies and image on one channel. By process of elimination I've been able to isolate the problem to the Parasound. What would cause such a problem?
 
Have you opened the amp up and done a visual inspection looking for discoloured resistors, sign
of capacitor stress (bluging, shrinkage or the plastic sleeve, leakage,,,). Is the amp original or refurb'd?
Maybe time to inspect solder joints, make sure alls well.

Could be dirty pots or switches. I wouldn't rule it out
Second that
 
You switch speaker cables at the amplifier side to make sure the problem follows to the opposite side speaker? Also switch the RCA input on the amp side to make sure the problem stay on the same side.
I am skeptical power amp just lost a little bit of highs. All the power amp circuits I saw are straight pass with no alteration of frequency response. That is most power amp has flat frequency response. I did see one case tweeter on one speaker blew and that side sounded flat.
 
You switch speaker cables at the amplifier side to make sure the problem follows to the opposite side speaker? Also switch the RCA input on the amp side to make sure the problem stay on the same side.
I am skeptical power amp just lost a little bit of highs. All the power amp circuits I saw are straight pass with no alteration of frequency response. That is most power amp has flat frequency response. I did see one case tweeter on one speaker blew and that side sounded flat.

Yes, I did switch the speaker cables and switched RCA inputs. I listened to preamp and cd player output through headphones, both of which are fine. I “think” it may have to do with the RCA to amp female connection. Sometimes I need to play around with the connection. Next step will be to do a detailed inspection of the amp.

Thanks
 
What do you mean by this? You get back the highs by jiggling the cables?
No, sometimes I have to jiggle the connection to get any sound in that channel.
Further clarification…after playing around with the tone controls it really appears that I’m losing more mid-range than high-end. On a certain recording I can’t even hear the vocals in the affected channel.
 
No, sometimes I have to jiggle the connection to get any sound in that channel.
Further clarification…after playing around with the tone controls it really appears that I’m losing more mid-range than high-end. On a certain recording I can’t even hear the vocals in the affected channel.
That is really strange, open it and see whether there is anything loose and look funny.

I google to try to find the schematic, the closest is the HCA-1000 schematic. Maybe it is close to yours. There is really nothing in the circuit controlling the frequency response. The circuit is very typical power amp with complementary input stage and 4 output transistor pair. Maybe yours have 2 pairs instead as yours is lower power. It is very similar to my Acurus except this one use JFET transistor as input. It has some DC servo control on this one.

It is most strange that you have scooped mids rather than highs rolling off. The only part in the circuit is part I circled in red. But those affect frequency at over 100KHz for stability compensation.

If you have the schematic, post it so I can look at it. Don't take for granted HCA-1000 is the same with just more output transistors.

HCA-1000.JPG
 
  • Like
Reactions: mbz
That is really strange, open it and see whether there is anything loose and look funny.

I google to try to find the schematic, the closest is the HCA-1000 schematic. Maybe it is close to yours. There is really nothing in the circuit controlling the frequency response. The circuit is very typical power amp with complementary input stage and 4 output transistor pair. Maybe yours have 2 pairs instead as yours is lower power. It is very similar to my Acurus except this one use JFET transistor as input. It has some DC servo control on this one.

It is most strange that you have scooped mids rather than highs rolling off. The only part in the circuit is part I circled in red. But those affect frequency at over 100KHz for stability compensation.

If you have the schematic, post it so I can look at it. Don't take for granted HCA-1000 is the same with just more output transistors.

View attachment 875391
Wow. Thanks for the great and detailed response. I believe the 750 is very close to the 1000. I hope to be able to inspect the board early in the week and I'll report back. I guess I should have paid more attention in class when I started towards a degree in EE, but eventually got a degree in Mechanical Engr.
Cheers
 
Wow. Thanks for the great and detailed response. I believe the 750 is very close to the 1000. I hope to be able to inspect the board early in the week and I'll report back. I guess I should have paid more attention in class when I started towards a degree in EE, but eventually got a degree in Mechanical Engr.
Cheers
If nessary, post pictures. Please confirm the schematic is very similar, then we can go deeper. But I have a suspicion it is going to be obvious. There's nothing in the design that can do that.

Ask as many question as you want, you have a second chance to get into EE by fixing this amp!!! I just designed and finished testing my amp and is similar in configuration as this but with a lot more to lower distortion. I am pretty sure I can answer a lot of your questions.( I hope!!)

I've been an EE for 30 years and retired, but I cannot stop designing as it's so much fun. I am doing it the hard way, design from ground up, layout all my pcbs and build it all myself.
 
Last edited:
  • Like
Reactions: mbz
If jiggling makes a difference, I would suspect the cables first and the connectors second. If it were me, I would get new cables and install them with a drop or two of Deoxit on them and run them in and out a few times to help apply Deoxit on the inside contacts.
 
If jiggling makes a difference, I would suspect the cables first and the connectors second. If it were me, I would get new cables and install them with a drop or two of Deoxit on them and run them in and out a few times to help apply Deoxit on the inside contacts.
I would also suspect the solder joint between the connectors and the board. I've seen a lot of these that need touchup.
 
Back
Top Bottom