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Old 15th October 2012, 06:41 PM   #31
BV is offline BV  Slovakia
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If is schematic in post 22 valid, I see here more possible problems:
Undefined ground potential at inpput (missing resitors from OA +Inputs to groud), missing input hf filtering (some RC lowpas at inputs, blocking capacitors..), missing series resitors (15-100R) in outputs, capacitive load (cable ..) can cause instabilty and oscilations-manifests itself sometimes as noise.. Output opamps are not needed, PGA internal opamps can drive relatively low loads (600ohm an more) direct.
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Old 15th October 2012, 09:43 PM   #32
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Quote:
Originally Posted by BV View Post
If is schematic in post 22 valid, I see here more possible problems:
Undefined ground potential at inpput (missing resitors from OA +Inputs to groud), missing input hf filtering (some RC lowpas at inputs, blocking capacitors..), missing series resitors (15-100R) in outputs, capacitive load (cable ..) can cause instabilty and oscilations-manifests itself sometimes as noise.. Output opamps are not needed, PGA internal opamps can drive relatively low loads (600ohm an more) direct.
So, you mean the following:
1. Some resistors from Input to GND, say 10k?
2. Input capacitors, as in caps in a series with the signal?
3. Resistors in series with the output, say 100R?
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Old 15th October 2012, 10:03 PM   #33
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I wouldn't assume that the setup with headphones automatically means that the problem is with the hypex amp or wherever one is assuming the noise is coming from.

With my implementation of the CS3318, I had it driving opamps in an inverting configuration that used the smallest resistor that the datasheet said the device could safely drive, this is something like 2.5k. Setup like that the system had something like 105dB SnR and when feeding the power amps + speakers resulted in a noise level that was far too high for comfort. I redesigned the output buffers to work non inverting instead and boom 120dB SnR and the problem was gone.

I mention this because another CS3318 also drove a headphone-driving, power-opamp, in the same general configuration, with the 2.5k resistors. This also had the same degraded SnR ratio, but through the headphones (HD650s) was completely inaudible. The reason why it was a problem through the power amp was because of the 30dB gain and 95dB drivers. The headphone amp on the other hand only had a gain of 2.

It could just be that the PGA/implementation is the culprit and is being noisy.

I don't know if this has been mentioned, but if another power amp, with gain similar to that of the hypex, could be tried instead, it would help to localise where the problem is.
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Old 15th October 2012, 10:11 PM   #34
maxw is offline maxw  United Kingdom
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Quote:
Originally Posted by 5th element View Post
With my implementation of the CS3318, I had it driving opamps in an inverting configuration that used the smallest resistor that the datasheet said the device could safely drive, this is something like 2.5k. Setup like that the system had something like 105dB SnR and when feeding the power amps + speakers resulted in a noise level that was far too high for comfort. I redesigned the output buffers to work non inverting instead and boom 120dB SnR and the problem was gone.
Which resistors are you talking about? In a series with the output of the PGA?

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I don't know if this has been mentioned, but if another power amp, with gain similar to that of the hypex, could be tried instead, it would help to localise where the problem is.
I would have done that on day one if I had another amp
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Old 15th October 2012, 10:26 PM   #35
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Which resistors are you talking about? In a series with the output of the PGA?
Here I am talking about my implementation with the CS3318. The resistors aren't in what you've got. The point, was that while my implementation was silent when used with headphones, it was noisy with the loudspeakers. Ergo, with your setup, just because the PGA implementation is silent with the headphones that doesn't automatically mean that it is going to be quiet enough to be used with your amplifier and speakers.
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Old 15th October 2012, 10:50 PM   #36
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So, you mean the following:
1. Some resistors from Input to GND, say 10k?
Yes, this defines the input impedance of the unit too. With this set too high or undefined you can get problems.

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2. Input capacitors, as in caps in a series with the signal?
Yes, these block any DC but might not be required. BV however was also referring to an RC network, with R in series and C to ground to form a low pass. This will prevent any high frequency grunge from entering the device that could cause problems.

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3. Resistors in series with the output, say 100R?
Absolutely, these are necessary to keep things stable, although whether or not you will get instabilities depends on the cables connected to the system, the load at the other end and then the opamp driving the load. The 100R effectively decouples the output from any parasitic reactance that could cause instability.
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Old 15th October 2012, 11:39 PM   #37
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Enough with the trolling, you're not really contributing to this thread!
You didn't have any contributions to this thread until I replied.
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Old 16th October 2012, 12:34 AM   #38
Bonsai is offline Bonsai  Taiwan
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I think all these suggestions are valid MAXW. So add the series output resistors, add the input bias resistors.

But, I think you still have noise problems from your Hypex units and SMPS. You only have noise when they are connected in h signal chain. You may need some screening and mains filtering to solve this.
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Old 16th October 2012, 06:56 AM   #39
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I doubt it's the Hypex amps + SMPS. I've tried various Hypex-based DIY amps, even next to phono stages, and they're extremely benign in terms of emissions. I've had more trouble with Class AB amps and their noisy toroids...
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Old 16th October 2012, 09:19 AM   #40
maxw is offline maxw  United Kingdom
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Originally Posted by Bonsai View Post
I think all these suggestions are valid MAXW. So add the series output resistors, add the input bias resistors.
OK I'll do that tonight and report back!
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