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Old 5th August 2007, 11:46 AM   #21
fotios is offline fotios  Greece
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Quote:
Originally posted by Tex Houston
Thank you again - I may now stick with the single supply as i can use the other one on a new project. The relays are after the input buffers yes but the volume pot is on the board directly after the relays and before the U105 IC - will this make a difference to the ideal pot value? There seems to be no noise directly after the input buffers, so I guess it's happening after that.
Tex, the value of pot it is right by anyway. Simply the most common place of it is after the main amplification IC, the AD797 in your case. This IC it is formed as noninverting op. amp. and the number of paralleled caps around it place me in thoughts. The quality of caps it is good? Why so many caps paralleled? You may search about this to the present and we will see furthermore. It is so strong heared the hiss?
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Old 5th August 2007, 02:06 PM   #22
AndrewT is offline AndrewT  Scotland
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Hi,
this pre-amp has the opamps wired for unity gain.
The 5534 are not suitable for unity gain. They should have the comp cap soldered in.
Change them for something more suitable.
It would take just one of these to oscillate and all subsequent opamps would do their best to amplify that noise and it will affect the final sound.

Who told you to fit 5534s here?
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Old 5th August 2007, 06:32 PM   #23
fotios is offline fotios  Greece
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Quote:
Originally posted by AndrewT
Hi,
this pre-amp has the opamps wired for unity gain.
The 5534 are not suitable for unity gain. They should have the comp cap soldered in.
Change them for something more suitable.
It would take just one of these to oscillate and all subsequent opamps would do their best to amplify that noise and it will affect the final sound.

Who told you to fit 5534s here?
In the application note AN142 from Philips, it is refered that the adding of a compensation capacitor of 22pf across pins 5 and 8 improves the bandwidth of NE5534 in voltage follower arrangement or for a gain less than 3 to 5. This only for reporting reasons from me.
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Old 5th August 2007, 11:33 PM   #24
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Default Alternatives?

Thanks again - the NE5534 was on the circuit diagram when I bought the PCB. Can you suggest an alternative or should I just solder the cap on the underside of the board. Do I have to do this on all the IC's? What about OPA134? The AD797's are very expensive.
The hiss I have at low levels is only audible when close to the speakers - but hey! it shouldn't be there at all should it! The preamp seems to sound very good apart from this.
One more question - what is the advantage of having more than one input buffer, when one could just switch signals beforehand and just use one? I notice there is a slight difference in component value for the Tuner input , but the DAC and CD are the same.
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Old 6th August 2007, 02:20 AM   #25
anatech is offline anatech  Canada
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Hi fotios,
For a preamp, the use of dual power supplies is a little silly. Your grounds will be commoned at every source and the power amp. You must take a systems approach to these designs. There isn't enough power draw to even warrant considering a dual mono design here.

Hi Tex,
Glad you found the problem. I would now try the proper op amps in this unit. It's worth the money in your application.

A buffer for each input would drive the volume control better, but you could use only one after your selector switch. This achieves the same thing. If you install a tape out, buffer this output also.

-Chris
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Old 6th August 2007, 08:22 AM   #26
fotios is offline fotios  Greece
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Quote:
Originally posted by anatech
Hi fotios,
For a preamp, the use of dual power supplies is a little silly. Your grounds will be commoned at every source and the power amp. You must take a systems approach to these designs. There isn't enough power draw to even warrant considering a dual mono design here.

Hi Tex,
Glad you found the problem. I would now try the proper op amps in this unit. It's worth the money in your application.

A buffer for each input would drive the volume control better, but you could use only one after your selector switch. This achieves the same thing. If you install a tape out, buffer this output also.

-Chris
Hi Chris
Generally the use of individual power supplies in preamplifiers it is not commonly used but it is a rule in expensive Hi-End devices. And the reason is that: If we have a source with individual ground conductor for each channel such as the phono cartridge, there is no problem of ground loop from a preamplifier with common supply - and thus a single ground node - for the two channels. Also if the source it is a very expensive SACD with individual supplies for each channel, again there is no problem with the same preamplifier. But in the case of the majority of CD or DVD etc. players where only one supply exists for the two channels simmultaneously, only one ground node exists common for the two outputs. If the preamplifier has a single supply for the two channels and thus again a single ground node for the inputs, the two shields of the stereo signal cable arriving from the source outputs they are in position to create a nice ground loop. According to the total gain of preamplifier this can heared louder or quiter. For this reason i thing the use of individual supplies for each channel - and thus seperate ground nodes - in the preamplifier it is the better solution.
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Old 6th August 2007, 09:06 AM   #27
fotios is offline fotios  Greece
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To understand you better my thought, i quote bellow a schematic diagram with the two cases.

Click the image to open in full size.
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Old 6th August 2007, 09:56 AM   #28
AndrewT is offline AndrewT  Scotland
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Hi Fotios,
a group of posters laughed at my suggestion for keeping the grounds between the channels in a pre-amp completely separate.

Were you reading that thread?

I would keep the channels separate, including the power supplies.

I have only built one passive pre-amp. It's only big downfall was hum at low levels, once I had optimised all the compromises that a passive inflicts.
That was nearly thirty years ago and I have never tried a dual mono.
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Old 6th August 2007, 09:56 AM   #29
AndrewT is offline AndrewT  Scotland
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Hi Fotios,
a group of posters laughed at my suggestion for keeping the grounds between the channels in a pre-amp completely separate.

Were you reading that thread?

I would keep the channels separate, including the power supplies.

I have only built one passive pre-amp. It's only big downfall was hum at low levels, once I had optimised all the compromises that a passive inflicts.
That was nearly thirty years ago and I have never tried a dual mono.
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Old 6th August 2007, 10:43 AM   #30
fotios is offline fotios  Greece
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Quote:
Originally posted by AndrewT
Hi Fotios,
a group of posters laughed at my suggestion for keeping the grounds between the channels in a pre-amp completely separate.

Were you reading that thread?

I would keep the channels separate, including the power supplies.

I have only built one passive pre-amp. It's only big downfall was hum at low levels, once I had optimised all the compromises that a passive inflicts.
That was nearly thirty years ago and I have never tried a dual mono.
Hi Andrew
First of all i am very glad to meet you again after a long time. I haven't read any thread about this. Simply i had distressed very hard in the past - before 20 years i think - when i was tried to build my first power amplifier a monster with +/-70Vdc supply. I have yet in my stock the transformer with the single secondary winding of 49,5-0-49,5 Vac which of i had tried the use in the common supply with the single ground node for the 2 channels. What a buzz! What a pity! This 800VA nicefull xformer in my shelf! You can understand furthermore why our thougts coincide.
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Fotios
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