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...
Thanks
Do
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...
Thanks
Do
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?????
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.
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.
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
@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
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
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.
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.
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
I am using EH 6922's for the pre, and a Ruby 6L6 J/J 12at7 for the psu. All of my tubes were only around a hundred bucks. I didn't want to invest too much yet, just in case....
Mine is mounted on a plywood board just like yours too. It looks like your tranny feeding the 12 at7 is left ac? On mine a made a simple 12 volt 7812 regulator to feed it dc. This might help. Also try to move the transformer further away. I am using iron core hammond 300 series. I grounded the psu to mains, and grounded my volume pot. This helped a lot with " noise".
Brian
Mine is mounted on a plywood board just like yours too. It looks like your tranny feeding the 12 at7 is left ac? On mine a made a simple 12 volt 7812 regulator to feed it dc. This might help. Also try to move the transformer further away. I am using iron core hammond 300 series. I grounded the psu to mains, and grounded my volume pot. This helped a lot with " noise".
Brian
Could be VHF oscillation. Are grid stoppers right on the pins, not half an inch away? Remember, the 6DJ8 was designed for high performance VHF receiver duty and it doesn't know you only want it to amplify up to 20kHz. Layout could be critical with this sort of valve, as any circuit loop could look like a resonator.
Sorry, I have not followed my own thread for some time...
My SP10 is perfectly working now. I had oscillation from the E88CC (6DJ8) tubes. I added grid stoppers to the first pair of tube near the input.
A tube guru I know told me that the tubes were oscillating and that 1K grid stoppers directly soldered on the sockets would be needed. Took his advise and did some more research on the issue as I wanted to understand but now it is a thing from the past. This preamp of really superb!
Also, the volume pot is better located at the output than the input according to my tests. This way it is totally silent.
Hi hobbit1979, I'm using several transformers, one Antek AS-1T200 for B+, also a 12.6Vac for filament of PSU and another transfo for SP10 filament.
Have fun with the project!
Do
My SP10 is perfectly working now. I had oscillation from the E88CC (6DJ8) tubes. I added grid stoppers to the first pair of tube near the input.
A tube guru I know told me that the tubes were oscillating and that 1K grid stoppers directly soldered on the sockets would be needed. Took his advise and did some more research on the issue as I wanted to understand but now it is a thing from the past. This preamp of really superb!
Also, the volume pot is better located at the output than the input according to my tests. This way it is totally silent.
Hi hobbit1979, I'm using several transformers, one Antek AS-1T200 for B+, also a 12.6Vac for filament of PSU and another transfo for SP10 filament.
Have fun with the project!
Do
Thanks for answering, I will consider your advice.
I'm going to use a transformer windings canterbury these specifications (hopefully worth because I have made to order):
Power rating: 147VA
Primary: 0-230V @ 50Hz
Secondaries: 195-0-195V @ 70mA rms + 2 x 0-15V@2.5A rms + 0-12.6V @ 2.5A rms + 0-63V @ 2A rms
A greeting.
I'm going to use a transformer windings canterbury these specifications (hopefully worth because I have made to order):
Power rating: 147VA
Primary: 0-230V @ 50Hz
Secondaries: 195-0-195V @ 70mA rms + 2 x 0-15V@2.5A rms + 0-12.6V @ 2.5A rms + 0-63V @ 2A rms
A greeting.
