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Old 1st August 2006, 06:20 AM   #21
nonoise is offline nonoise  South Africa
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Whilst composing the previous reply you seem to have reached a similar conclusion
I have to agree with your opinion of the soundís character via the direct output option.
The gain is unity though so it will sound softer than via the active outputís
The original output buffers are absolute rubbish IMO. Replacing them could only help
The only problem is that, if memory serves the power supplies were uni-polar most of the good op-amps are happier running off a bipolar (split) power supply.
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Old 1st August 2006, 07:17 AM   #22
AndrewT is offline AndrewT  Scotland
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Hi,
Quote:
power supplies were uni-polar
this would explain why there are so many caps installed all over the board.

If you are right, then the designer seems to have made a decision to save on the PSU and AC couple all the stages.
I suspect his solution was not cheap.

If you plan to add a buffer for the pre output then consider a discrete opamp. They are reputed to sound a lot better, even though the specs do not match a good quality, expensive opamp.

5532 being a dual is a bit compromised for two output buffers and worse if you decided to bias the output stages into ClassA.
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Old 1st August 2006, 07:35 AM   #23
hoshy is offline hoshy  United Kingdom
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Hi,

Yes, my preamp has the two sets of outputs also - buffered/direct. After listening to both I think the buffered outputs really do sound better. So, I'll need to do something about the situation. I believe you're right about the single supply.

I had hoped that the MC circuit opamps would have had a split supply but unfortunately they don't. The transformer is external, but not the entire power supply. If brings in AC ~30v and that gets rectified on the main board. I would imagine it's possible to generate a split supply quite easily from this?

The other option is simply replace the existing buffer transistors with something a bit flashier. the buffer circuit (from the top of the board at least) looks like a transistor, a resistor, a poly cap and an electrolytic so it should't be hard to replace them all if needs be. I think I'd rather put a niceish opamp in though.

Correct me if I'm wrong, but I think you could run the opamp from a single supply, biasing the input (I did this in a guitar effect pedal). Though, would the DC blocking cap have an effect on the muscialness?
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Old 1st August 2006, 07:45 AM   #24
hoshy is offline hoshy  United Kingdom
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Quote:
Originally posted by AndrewT
If you plan to add a buffer for the pre output then consider a discrete opamp. They are reputed to sound a lot better, even though the specs do not match a good quality, expensive opamp.
Thanks Andrew,

I'm in the process of buying a Marantz CD63 (that I also plan to modify ) would copying the HDAM circuit be a good starting point? The HDAM is supposedly a discret opamp circuit.

Ashley
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Old 1st August 2006, 08:40 AM   #25
AndrewT is offline AndrewT  Scotland
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Hi,
you can run a chip opamp or a discrete opamp from single supply rail.

But you need to block any unmatched DC levels from stage to stage. Which is why you have all those 1uF and 220nF caps around the PCB.

A simple EF buffer should work and fairly quietly at that.

I think you should try replacing the transistors with new equivalents, just in case they have been damaged in the past. The outputs could have been shorted and upset the transistor characteristics.
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Old 1st August 2006, 08:45 AM   #26
hoshy is offline hoshy  United Kingdom
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Thanks again Andrew,

I'll pop the old transistors out at the weekend and see what I've got to replace them.
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Old 1st August 2006, 11:07 AM   #27
sreten is offline sreten  United Kingdom
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Hi,

Just add that for a CD players its hard to beat no circuitry
at all for performance versus cost. Depending on the topology
of the pre-amplifier and the value of the volume potentiometer
(which could be reduced and upgraded at the same time) and
keeping the cabling short it should be possible to make the
CD input / output entirely passive and better than the buffered
output.

Changing the transistors in the buffer will not change its
inherent linearity (assuming its as simple as stated) but
depending on the circuit noise may be improved.

/sreten.
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Old 1st August 2006, 11:23 AM   #28
hoshy is offline hoshy  United Kingdom
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Thanks. Noise was the first issue. Now I'm using the direct outputs they are quiter on the noise front but just don't sound as good. I'm going to try swapping the transistors first, to see how that affects things. then perhaps I'll build an opamp buffer.
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Old 1st August 2006, 11:31 AM   #29
sreten is offline sreten  United Kingdom
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Hi,

Without a full overview of the topology and the volume pot value its
hard to be accurate. Fully passive should be your best option for CD.

/sreten.
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Old 1st August 2006, 11:40 AM   #30
hoshy is offline hoshy  United Kingdom
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I'm talking about the preamp outs being active or passive. Not the inputs, so I see what you're saying that in terms of CDP to pre connectivity, less is more so to speak. But the transistors I was on about changing are for the outputs, I don't see that is would make any difference to the input. Or am I missing something??

Thanks again
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