SEX Power Supply Question (V2.0 vs V2.1)

[Natural Sound]

I have a SEX 2.0 amp that I built some time in 2005. It has MagneQuest "pinstripe" output iron, plate chokes, upgraded signal caps and C4S. An upgrade kit that at the time was referred to as "Enhanced SEX."

In another thread Paul Joppa talks about the limitations of the stock output transformer in SEX version 2.0. Then with the introduction of SEX 2.1 a new output transformer and plate choke was introduced... "This removed the technical limitations, so the power supply was simplified to a pseudo-dual-mono without the split rails."

Can someone expand on that for me? What are the benefits of a pseudo-dual-mono without the split rails?

Assuming a SEX 2.0 has upgraded iron, could it be modified to a pseudo-dual-mono like in SEX 2.1?

Thanks in advance.

 

[jrebman]

The power transformers are different and the transformer in the 2.1 kit only has one high voltage secondary, as opposed to the bifilar wound, double high voltage secondaries of the 2.0.

My guess wouldd be that most of the sonic improvement (aside from the iron) is that there are far fewer parts and connections in the 2.1's power supply and I'm guessing that the 2.1's power supply layout is a bit simpler as well.

Hope this helps,

Jim

 

[Paul Joppa]

The old Speco transformer was not rated for DC voltage on the winding, specifically the secondary. I didn't want to put any such voltage on that winding, because an internal short would put the voltage on the secondary which is connected to headphones. So, in order to ground one end of the transformer we would either have to keep the cathode bypass capacitor in the output current loop, or get the bias another way. I chose to do the latter, using the WE split-rail circuit. That in turn required that there be two independent power supplies, since you can't count on old-stock television tubes to come in matched pairs(!).

The double power supply was technically cool, but the bifilar power supply winding was at risk of internal shorts unless used in specific ways with the right orientation of terminals. That made it unsafe to offer the transformer by itself, which was a frustration to us and to a few customers who wanted to experiment with it. As we started using the same transformer in other products (Foreplay, Crack, and a couple more possible new products) the situation got more frustrating.

The new output transformer is rated for 500vDC on the primary, so there is no problem having bias voltage (20v) on the primary and grounding the secondary; therefor the dual power supply is not needed and a simpler power transformer was developed; it's in the SEX and Tode now and is being deployed into other products as we revise them.

The SEX 2.1 has a "pseudo-dual-mono" power supply, where the last section of filtering is split into two parts, one for each channel. SEX 2.0 has real dual-mono power, which gives slightly more separation of channels - though I don't really think the difference is audible in this circuit. The smaller parts count in version 2.1 prevented a larger price increase, which was important to us.

I've simplified above but I hope that helps!

 

[Natural Sound]

Thanks guys! Its been a while since I had the SEX amp playing music. Last week I took my FPIII out of the system to do a few upgrades. Not wanting to be without music I pulled the SEX amp out of a box in the closet and hooked it up to my speakers and my linux music server/DAC. I forgot how sweet this amp sounds. Right now I'm listening to Hank Jones, Christian Mcbride and Jimmy Cobb, West of 5th, 24bit 88khz. All I can say is holy crap!

I need to get a set of Orca's from Clark, set them in my home office and pair them with this amp.