Converting SRD-7 to pro voltage?

[Blueman2]

spritzer or others,

I cannot find a working link to the bias circuit.  I have been unable to find an SRD-7 pro so wanted to buy a regular SRD-7 and convert it to pro.  Can anyone PM me the circuit or help me find a link?  Thanks,

Blueman2

[mwl168]

Here is a link to a SRD 7 MK II schematic which has a Pro bias circuit: http://cdn.head-fi.org/5/5d/5d5004f8_srd7mk2diagram.jpeg 

A couple of modifications  are needed( thanks to Birgir! ) which was discussed earlier on this thread and includes information on obsolete parts replacement. I converted my SRD 7 MK I to a MK II based on these information. It worked very well but I have not used it since I built my KGSSHV. 

Good luck! 

Edited December 26, 2015 by mwl168

[Blueman2]

Doh!  I had that schematic but did  not understand what to do with it.  So I need to breadboard D101-106, C101-106, R101-105, and the back-to-back 100V zeners.  

 - I can either leave out C108 or put it before R104, right?

-  can I leave out C107 and R103 since I only want Pro bias

 Thanks!!

 

 

 

Edited January 16, 2016 by Blueman2

[mwl168]

The photo attached is not accessible. And don't forget to change the ballast resistor to 4.7M. I think I used a 1/2W, Mouser part # 594-VR37000004704FR5.

[Blueman2]

Finally getting around to changing my SRD-7 to Pro bias.  I noticed the part surge suppressors (ZL01-ZL04) are not in the original non-pro version.  I assume these are only there for accidental overload of the transformer, to protect it and the earspeakers?  If so, I will just take the risk and be careful not to have the volume too high.  Not sure where to find a similar part in any case.  

Here is the pro version schematic with the ZL01-04's:

Edited January 24, 2016 by Blueman2

[Blueman2]

Well, I built the circuit, but something is amiss.  Measuring between the CT of the transformer (end of R105) and before the final 4m7 resistor (just after D106/C106) I am getting only 400V.  Starting from the beginning of diode ladder, I get 80, 175, 250, 320, 365,400.  Another person (RDK845) reported the same result when he built the circuit, in the "DIY transformer box thread" on another site.  I am using a Fluke 77 IV to measure. 

Also, I tested the voltage between each diode, and it was between 94-97VDC across each diode, as expected.  When I added up the 6 readings, I get ~580V, again as expected.  But then when I measure from start of D101 to end of D106, I get only 420V. Really weird.  Shouldn't DC volts add?

Unfortunately, the other person who reported this never replied back with a solution.   

spritzer, I noticed you were involved in the other thread on the other site. Any insights into why I am getting these low reading??

Edited January 24, 2016 by Blueman2

[Blueman2]

SUCCESS!!!

It was pointed out to me (thanks sorenb!!) that the voltage measurements cannot be done from a typical digital voltage meter due to the impedance of the voltage meter itself. So I just installed my circuit into the non-pro SRD-7, hooked it up to my Integra ADM-20.4 power amp, and it sounds WONDERFUL!!  This was the first time I was able to listen to my new SR-507s, and I am completely blown away.  The bass is perfect.  Incredibly powerful but very clean.  I am hearing things I have never heard before in music I have listened to for years.  I really am stunned by the quality.  

Thanks mwl168, spritzer, sorenb, JimL and Dr. Gilmore for the advice and direction on this successful upgrade.  Now on to building my SRX-Plus amp to see how that compares.  Not sure how much better the sound quality can get, but I can't wait to see.  

Edited January 24, 2016 by Blueman2

[Blueman2]

I bought an SRD-7 Professional that I wanted to give to my brother so that he can experiment with Stax headphones.  These already have pro bias, but I was still going to re-wire it so that it has 4.7M final resistor on the bias.  Should I add 5.1K resistor on the signal outputs while I am at it?  I see most amps have these yet these SRD units do not. 

[spritzer]

The SRD units predate the output resistors by about two decades.    You could add them but I wouldn't bother. 

Do check the bias supply and see if there is a cap after the ballast resistor.  If so...remove it. 

[Blueman2]

Thanks, Birgir.  Is it acceptable to just add another 2.2M or even 4.7M resistor after the cap?  It would be easiest to just insert that into the line going to the bias pin and then leave the PCB alone.  .    

[spritzer]

Just remove the cap and replace the resistor. 

[matthew-levi]

I have an Stax Lamda and SRD 7 combination. Recently I opened up the SRD 7 to take a look inside and found the two transformers quite anaemic.I thought may be the performance of the SRD 7 could be better if we use a better transformer. Looking the schematic, it looks look the transformers are like tube output transformers. Does anyone have any idea about what spec those transformer would be like, primary impedance (facing the loudspeaker output from the amp) vs secondary impedance (facing the stators). Would a 10K : 8ohm push pull output transformer be suitable for this. Any advice would be appreciated.

[Blueman2]

I recall some threads on people upgrading the trafos on SRD-7s, but not sure how much of a difference it made.  But wow, I am amazed to see how much these SRD-7s are going for on eBay these days.  I bought 4 of them a couple years ago for $100 each.  Now going for 3x that or more.  

[matthew-levi]

Does anyone has any experience in upgrading the trafos on the SRD 7 and what were the results. Interested to hear any experience out there.

[dingding123]

Just curious,has anyone tried using dynahi to drive a srd 7?