[Uncle Bob]

Finally got some time to finish off my SohaII but I've hit a problem

I've got a very narrow adjustment range of the output voltage of the TL783, I can adjust only from about 113V to 115V from end stop to end stop on the P1P trimmer. This is a 10K trimmer as specified. The voltage at J3 reads 117V.

I may have damaged the TL783 as it has been desoldered and resoldered once, this would be a real pain as these devices are difficult to buy here in the UK.....

 

[bmwpowere36m3]

Quote:

Originally Posted by Ferrari /img/forum/go_quote.gif Looks like you are using Vishay Dale RN60D for R7L/R (620Ω). Was it not available as RN55D? or resistors from other brands (BC, Panasonic, Xicon) with the same size as RN55D? Nice start, BTW.

Yea, mouser was out of stock of RN55, so I just went with a RN60... to keep all resistors VD's. Thanks.

 

[runeight]

Quote:

Originally Posted by Uncle Bob /img/forum/go_quote.gif Finally got some time to finish off my SohaII but I've hit a problem I've got a very narrow adjustment range of the output voltage of the TL783, I can adjust only from about 113V to 115V from end stop to end stop on the P1P trimmer. This is a 10K trimmer as specified. The voltage at J3 reads 117V. I may have damaged the TL783 as it has been desoldered and resoldered once, this would be a real pain as these devices are difficult to buy here in the UK.....

Well . . . . . . . is there any way that you can get a replacement? I think you assessment is correct.

 

[bmwpowere36m3]

some more progress...

 

[wiatrob]

Quote:

Originally Posted by Uncle Bob /img/forum/go_quote.gif I may have damaged the TL783 as it has been desoldered and resoldered once, this would be a real pain as these devices are difficult to buy here in the UK.....

UB, I believe I have a spare I can send, PM me if you are interested... <EDIT> Yup, I have one...

 

[Uncle Bob]

Bill

Thanks for your kind offer but I've found a seller on ebay. He's charging top dollar but needs must so I ordered one last night. With any luck it will be here before xmas.

 

[bearmann]

Hi everybody!

I'm very, very interested in the SOHAII, but I don't know if it'll fit my needs.
I have a pair of Audio Technica W100, which are rather efficient and driving them with a CK²III (Gain 9) was quite impossible.
Now I'm curious how they would perform with a gain around 12?! (which is the gain of most tube combos inside the SOHAII, afaik)

Is the SOHAII capable of driving efficient headphones or should I stick with another project?!

Thanks a lot and best regards,
bearmann

 

[bmwpowere36m3]

I just finished assembling the PCB and have started to test the circuit out... TL+, TL-, and TH check out. TB+ is ~122V, is that okay, the setup says +103V?

 

[runeight]

Yes, I think that's ok. The B+ will be a little high when there is no load. I'm assuming that no jumpers are jumped right now. When you get the buffers going their load should pull the B+ down considerably and then the B+ into the HV regulator should be ok so that you can connect J3. But check this at each step.

 

[bmwpowere36m3]

Quote:

Originally Posted by runeight /img/forum/go_quote.gif Yes, I think that's ok. The B+ will be a little high when there is no load. I'm assuming that no jumpers are jumped right now. When you get the buffers going their load should pull the B+ down considerably and then the B+ into the HV regulator should be ok so that you can connect J3. But check this at each step.

Yes, I haven't jumpered anything yet.

 

[runeight]

OK. I think you're good to set up the buffers. It might be good to recheck all resistor values before you jumper J1 and J2. Then go for it.

 

[holland]

Quote:

Originally Posted by bearmann /img/forum/go_quote.gif Now I'm curious how they would perform with a gain around 12?! (which is the gain of most tube combos inside the SOHAII, afaik) Is the SOHAII capable of driving efficient headphones or should I stick with another project?!

About 9 with a 12AU7. More with different tubes.

All amps are capable of driving efficient headphones. FWIW, I use a 24-step attenuator, with a 2Vrms source. With my efficient headphones, I'm around 2-3 clicks, maybe 4 max for current production music. For stuff recorded around 89dB max, I'm about 4-5 clicks. With a 1Vrms source, I can probably extend another click or two.

There are things you can do to reduce the gain, but it may be negative, depending on what you prefer. One thing is increasing the amount of negative feedback. Another is using a finer grained attenuator.

It's entirely up to you, but I'm not sure you'll find many tubes or hybrids that are unity gain or a gain of 2. While it would be nice for another builder, you might be better off looking for a tube buffer, if you're looking for tubes.

If you've ever played with gain, you might be surprised how little you really need for efficient headphones. I think for some of my high sensitivity headphones, the last time I measured, I really only need a range of 500mV rms swing and a current buffer to have good use of the volume knob.

 

[bearmann]

Thanks for the statement, holland. I think I'll stick with a non-tube low-gain design.

best regards,
bearmann

 

[wiatrob]

Quote:

Originally Posted by m0b1liz3 /img/forum/go_quote.gif Wish I had bought a prototype kit earlier!

FWIW, you probably would have spent MORE on a prototype

 

[rds]

Quote:

FWIW, you probably would have spent MORE on a prototype

Lol, yeah. Or spent something on prototyping nothing.
Anyways, some day soon my production board will be showing up and it will all be well worth it I imagine.