SR45 amplifier

[johnsonad]

Thanks Paul, I'll take a chill pill and await the new design 

[Jim R.]

Paul,

So, will any of these new boards work in the old sr-45?  I already know the answer regarding the soft-start board :-), but interested in backwards compatibility to the existing design.  In the case that that won't work, can I transplant all the nice iron (nickel tfa-2004 jr, dowdys, and bpc-16ni to the new design?  I'm assuming you're designing for the s.e.x./stereomour output transformers and of course I'm wondering if there is enouugh wiggle room in the operating points to work with the 3k of the TFA-2004.

Best for me, of course would be to be able to use one of the new driver boards in the old existing design.

-- Jim

[Paul Joppa]

Jim-

I had to spend several hours on ferries today, and used the time to look at alternatives.

Right now, what appeals to me the most is a completely fresh design. It always bothered me that we made so many compromises to get away with existing parts and layouts, in what was (IMHO) our best-sounding amp - the topology was way ahead of the rest of the design (AC heater, no time delay, etc.) I kind of hate to do this, I always liked the idea of growth paths, but the original WAS a growth path, compromised to work with the original Paramour II, and it just didn't sell enough to make economic sense.

The original was an experiment; only a few were made and the manual was nearly non-existent. I learned quite a bit from that experiment; among other things I learned that this is never going to be a big seller. Even if I come up with a prototype that sounds great, I have doubts that Doc B will want to develop a full-scale manual in the traditional format - that's a LOT of work and has to be spread over many units to make sense. It may only makes economic sense as an already-built amp.

So I am thinking of a much larger circuit board to house the power supplies - including regulated DC on the filament and heater, as well as the shunt-regulated high-voltage power. The manual for PC boards should be simpler to do. I'd like to do it with all film caps (except for the heater power, there are no polypropylene caps at 10,000uF!), and include time delay relays for high voltage and for shorting the OPT to prevent core magnetization. And impedance switches and an input level adjust for channel balance - the whole nine yards.

The MQ iron would still be a reasonable choice for the operating point, and the chassis plate may or may not be the same - that partly depends on whether or when we update the Paramount chassis. Of course, the Dowdy choke won't fit on any of our chassis plates anyhow ...

Of course, this is not set in stone yet. If there's an outcry for something more Bottlehead-y and less high-end, I have a few ideas. I'm just thinking that only a relatively few people will be into it so deep as to want a spendy 2-watt amp.

[johnsonad]

Thanks PJ for the update. Hopefully others will chime in but I for one look forward to your 'high end' design. May they be the sweetest 2 watts available    

When you set the primary impedance will you please let us know?  
Regards,

Aaron

[Paul Joppa]

I'll use the SEX/Stereomour output iron at 4K primary, so the load impedance will be 2-4-8-16 ohms. That's been central to the plan all along. It has a generous magnetic headroom at this operating point, and most listeners - including myself - have been very happy with it. The Magnequest BH-5 at 3K is still suitable, as is the BH-2 plate choke, and I'll try to leave room and mounting holes for them.

[Jim R.]

Paul,

Thanks for the detailed reply and explanations.  I think, given what I have and have invested in, it makes most sense for me to just continue on the path I'm on, perhaps design and build a more substantial heatsink for the existing driver arrangement/shunt reg board, and my larger chassis.

I will however possibly look into the opt shorting switch/relay, but realizing that would require an additional power supply that may or may not impact noise levels.  Also, given my speakers' sensitivity and the existing reports on the amps noise performance, I'm not sure how much, in reality, I'd gain in sonics over a fully maxxed out version of the existing design -- vcaps, copper chassis plate, dowdy, foam FEP OCC wire, etc.

After all, it's still going to be 2 glorious watts! :-)

-- Jim

[xcortes]

I have three pairs of TL-404s waiting for this project to go forward (it took me many years to get a hold of them three).

In the meantime I'll build a First Gen SR45 with the PT-2s, Dowdys and Cobalt pinstripe 2004 Jrs. For the WE's, that is.

[Jim R.]

Xavier,

I want those cobalt tfa-2004 jrs! :-)  I wish there could be another run of the cobalt iron from MQ.  Not that the iron I have on hand and on order is anything to sneeze at, and will probably be the best amps I've ever owned, but still... :-)

-- Jim

[johnsonad]

Xavier,

I want those cobalt tfa-2004 jrs! :-)  I wish there could be another run of the cobalt iron from MQ.  Not that the iron I have on hand and on order is anything to sneeze at, and will probably be the best amps I've ever owned, but still... :-)

-- Jim

I have first dibs Jim!  Sorry my friend 

[xcortes]

Hmmm,  maybe I should start an auction?

Sorry pals. You're gonna have to come down to Mx to listen to them!

[Alonzo]

So is the general opinion that the old SR-45 should be shelved in favor of the the upcoming boards?  I happen to have the orginal kit and a couple of PT-2 transformers that I was going to try to make music with soon.  Will there be a significant improvement from the old amp to the new?  Are the newly proposed boards ready for production or behind other more profitable kits (just trying to see how far down the "to-do" list these are for Paul)?

[Jim R.]

Xavier,

Ok, and I'll bring the Hacienda de Chihuahua :-)

Alonzo,  I've thought about this too, but at least for now I'm going to stick with what I have as I believe it will be a fine amp and I'm just not sure I will really need the DC filament supply.  A great amp is still a great amp :-)  Of course this is just my approach and for now, but I'll keep an open mind for the future if it turns out to be a significant enough upgrade.

-- Jim

[Paul Joppa]

The new design has two main features - DC filament power for lower hum, and a bigger heat sink for better reliability. It may have film caps for reliability as well; that depends on where Doc B wants to position it, which depends on who we expect to be the customers. If the hum is no problem (it seems to run around 1mV with good 45s, which is actually quite low for an SET) then there would not be a great deal of sonic difference.

The other difference is the OT-2/PC-3 output transformer and choke, which is better matched to the 45 than the original Magnequest iron. I'm not saying my design is better than Mike's (!), just that a 4K load gives better damping than a 3K load.

As for timing, we recently had a great brainstorming session to look at new product development for the next year or two. We haven't documented the results or made all the decisions yet, but we do have a general idea of where we want to go. I can't be very specific yet, but now that the BeePre design is pretty much finalized, the DAC is the next priority. My involvement is just the analog tube output and power supply and won't be so all-consuming, so I'll probably work on the SR amp at the same time.

[xcortes]

Thanks Paul. Sounds great. That time frame will allow me to save some money for my three pairs (did I mention I already have three pairs of TL404s waiting for this to happen?). I like the 4k better than the 3k because it's closer to the 5k of my 404s. I know you like 404s.

Jim: We're ready to have you here. No need to bring the tequila. I have some

[johnsonad]

I am keeping my fingers crossed  for the film cap PS. Would it be too much extra to design two different PS boards, one for film (with a choke or two) and a standard CRC supply to cover both bases?  Or possibly making the film supply schematic an option and we could point to point a PS for it in a none standard chassis?  The film supply board would find nice cross coverage as an upgrade path for the Paramounts too.

Just brain storming here is all  . It seems in the classic BH spirit people intend to customize the design regardless. Mine will put a pair of Dowdy's to use and an oddball OPT for Stax headphones. Thanks PJ for keeping this project alive!