Audio Research SP-10 Clone HELP please - diyAudio
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Old 6th April 2012, 04:42 AM   #1
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Default Audio Research SP-10 Clone HELP please

Hi,

First I would like to say that this preamp will surely sound really good since it already does even with the issue I'm having...

I built this preamp about 2 years ago since I wanted to familiarize myself with tubes. Had a couple hiccups and made everything work but a humming sound was there and I don't believe it was ground loop. I couldn't figure it out so I let it sit there collecting dust for a long time...

I recently started to go back on that SP-10 preamp when I had spare time at night. I've removed every AB resistors since they were old and non matching. Some of the values were really off... I've installed PRP resistors all over. The sound is better and the "shhhhhh" sound that was really heavy on one channel is gone. I still have some of it in equal strength on both channels but much lower. The noise floor is still not quite there. I was thinking it could be the tubes I used in the power supply so I'm ordering better ones. It uses 6L6 and 12AT7 in the PSU. The 6L6 that I have now is a EH and the 12AT7 is EH as well. The ones I'm getting are Tung-sol 6L6G (Big Bulb) and Mullard CV4024 for 12AT7. Don't know if this will help reduce the noise floor...

One thing that bothers me is a "hum" issue, maybe a ground loop but doubtful... There are two 0.47uF caps between the 6922 tubes. Whenever I put my hand near them 2"- 3" it starts humming much louder, touch them and it is really loud... I've tried different caps there (Solen, Dayton, Obbliggato...) but they all do the same thing. If I don't put my hand close to the caps, I do get a lower hum that goes almost away when I touch the signal input ground with my bare fingers while music is playing... Go figure...

Now keep in mind that it is still on plywood for prototyping but I would appreciate your help on what to try next...

Here's the PSU schematics but I don't have the preamp's just the BOM...

SP-10 psu.jpg

Thanks
Do
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Old 6th April 2012, 06:28 AM   #2
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Quote:
I was thinking it could be the tubes I used in the power supply so I'm ordering better ones.
Very unlikely this will help. The 12AT7 and the zener may be contributing a bit to noise but once it passes through the opamp regulators it will be reduced below audibility.

There is a pretty good chance the circuit around the opamps is oscillating though.

Don't you have a scope?

It may be a good idea to quickly build a passively filtered PS for comparison purposes. Not sure if your transformer has enough voltage output but some simple RC filtering will give you a pretty good PS for experimenting. This will eliminate any doubts about the regulators - both tube and solid state.

As for the capacitors injecting hum when you touch them, well, don't That's what capacitors do in high impedance nodes.

Your hum issues will be hard to troubleshoot if there is no screening around the preamp. If sceening doesn't help it may well be a ground loop or oscillation.

But why on earth build an SP10 clone?????
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Old 6th April 2012, 12:12 PM   #3
DF96 is offline DF96  England
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I assume you have the usual 'big audiophile' caps? These have a large surface area and so have a large capacitance to nearby sources of hum, including you. Try smaller value caps and/or smaller physical size caps in a screened box, well away from any AC.

This problem of big caps picking up hum seems to be getting more common. People seem to think that bigger must be better. Not always true. Big caps can mean hum, and problems wth subsonics getting through.
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Old 6th April 2012, 03:57 PM   #4
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I will post pictures of the setup soon and hope this will help.

@analog_sa
nfortunately, I'm not one of those lucky ones to own a scope... I need to go to my friends at Simaudio for that and since they're extremely busy, I cannot make it that often... By screening, you mean a grounded chassis?

As for the SP-10, I'm a newbie for tubes (I know my SS very well). I wanted to try something and there was a PCB available at Jims Audio so I tried it. What would you recommend?

Lately I have found the Aikido for Glassware Audio Design... Don't know if they're good and which one to use as preamp to match with SS amplifiers and which tubes to use for impedance matching with the gears... If you can direct me to an extremely good sounding preamp project, I can handle it. I don't mind using point-to-point either.


@DF96
I had some 0.47uF Obbliggato caps which were big and decided to use the Dayton smaller caps but with same results. I have even smaller ones to try so I'll do later on.

Guy's if you believe I'm wasting money injecting in that SP-10, let me know but also tell me what could be a very good alternative.

Of course I've put in lots of money already in that SP-10 and would love to make it sing perfectly but if there's something better that I can reuse my 4x Mullard E88CC tubes it would be great. They're 200$ for 4 of them... Don't want to waste the money.

Thanks
Do
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Old 6th April 2012, 04:29 PM   #5
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Quote:
Originally Posted by pinnocchio View Post

As for the SP-10, I'm a newbie for tubes (I know my SS very well). I wanted to try something and there was a PCB available at Jims Audio so I tried it. What would you recommend?

Lately I have found the Aikido for Glassware Audio Design... Don't know if they're good and which one to use as preamp to match with SS amplifiers and which tubes to use for impedance matching with the gears... If you can direct me to an extremely good sounding preamp project, I can handle it. I don't mind using point-to-point either.

Guy's if you believe I'm wasting money injecting in that SP-10, let me know but also tell me what could be a very good alternative.

Thanks
Do
A SP-10 cost a fortune in tubes, and power supply is troublesome

Go with a CAT (10 tubes w/phono) or others modern design
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Old 6th April 2012, 04:54 PM   #6
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Quote:
Originally Posted by Praudio View Post
A SP-10 cost a fortune in tubes, and power supply is troublesome

Go with a CAT (10 tubes w/phono) or others modern design
My SP-10 clone only uses 4x 6922. There is no phono section on it.

Pardon me my ignorance, what is a CAT? I don't need the phono section since I don't own any LPs and do not plan in the long run.

My Source is a Buffalo II with WaveIO and my I/V is dual Tube-I-Zator from DVBProjekt for balanced mode. I'm using TJ Full Music 12AX7 in the Tube-I-Zator and it sounds fantastico!

Thanks
Do
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Old 6th April 2012, 05:09 PM   #7
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Here's the pictures.

You'll see the 2x 0.47uF Dayton caps is where the hum comes from when approaching them with my hands.

sp10-1.jpg sp10-2.jpg sp10-3.jpg

sp10-4.jpg sp10-5.jpg

The board has not been cleaned yet, still in prototype...

Thanks
Do
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Old 20th September 2012, 11:18 PM   #8
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Wow, very nice and interesting project! I am planning to use the same boards for my next project. Have you made further progress Pinnoccio? Are the boards okay?
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Old 20th September 2012, 11:55 PM   #9
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Hmmm..From your explanation, it seems that hum is radiating from pcb copper trace of input tube. either from pin2 or pin7..

You can eliminate this hum by cutting the pcb trace and run a shielded wire instead to the gain control..the same thing with 0.47 copper trace to the grid of tube..hope this help.
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Old 21st September 2012, 12:20 AM   #10
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Quote:
Originally Posted by Kaplaars View Post
Wow, very nice and interesting project! I am planning to use the same boards for my next project. Have you made further progress Pinnoccio? Are the boards okay?
Actually, another forum member contacted me recently (Birder). He said he had build the same project but his was dead quiet. Could also be the component choice and the tubes used. He gave me a few pointers which I haven't had a chance to try yet.

It sounds really good but I have to get rid of the hum. I think I will try Ricdelros's tips. Seems like Birder didn't have to go this route though...

@Birder, could you please let me know which tubes you use (brand model) in the PSU and preamp? There's a few resistor values I would like to go through with you if you don't mind?

Thanks
Do
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