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Originally Posted by mirumu
 One thing I notice in that circuit is the center tap which I assume is there for the purpose of creating a ground reference shared between the bias supply and the audio signal. In my SRD-7 Mk2 I get some 50Hz mains hum that is quite noticeable with my SR-X Mk3 (The O2s don't seem to be efficient enough for it to be very noticeble). There is no direct connection to the ground in the mains plug, it's only got two prongs. Considering these center taps are the only place the audio and bias signals interact I'm assuming that the hum is creeping into the signal through here. Is there a way I can minimise this? Perhaps running a line from the mains ground to this spot (i.e. on the center tap PCB traces) in the circuit to actually make it a true reference to ground?
On another semi-related note, I am planning to build my own custom transformer box along the lines of Andrea Ciuffoli's circuit but with a substantially higher ratio for the two signal transformers so it can be used with lower power amplifiers (under 10W). Now having more turns will increase the capacitance. I'm wondering if this will have any effect other than reduced high frequency performance? I'm thinking that as long as the transformer can maintain good performance (under +/- 1db) to near 25Khz or better this is not a concern. If it was a problem it would seem I'd have to scrap the idea and settle for a lower ratio. |
The SRD-7 Pro does not use a transformer in the bias supply and is not isolated from the AC line. It also does not use a common ground. The primary and secondary of the step-up transformer and bias supply are floating to ground. While listening to the headphones (but no music playing) quickly unplug the unit from the AC outlet and see if the hum goes away. If it does you may want to try a few ideas: Try putting a ground connection to the chassis (metal box). Try the polarity of the AC plug. Try a isolation transformer on the AC line. If all this fails there may be a problem with the SRD-7 Pro.
If you get or custom order a transformer a 24:1 turns ratio (like Stax uses) is getting close to the limit. When the turns ratio gets to high it starts to degrade performance. One other way to get higher voltages is to use two transformers in tandem. Parallel the primaries and series the secondaries. That’s what Quad, Martin-Logan and Audiostatic do.
Hope this helps,
AudioD
