Playing in the Sandbox

[debk]

Ok I know to some it is sacrilege to play in the sandbox.  My latest project is a Pass Lab F4 clone.  It is an interesting amp since it has unity voltage gain.  I want to drive it with a SET amp.
So far I have built one channel in a parallel mono configuration.  In this configuration it puts out about 50W.
Source--->Extended Foreplay III preamp modified for 6SN7's---> Paramount 2a3 amp --->F4 in parallel monoblock configuration.

I tried both the 8 ohm and 16 ohm tap on the output transformer and listened to both with a variety of resistors across the Paramout output (12R, 15R, 20R, 22R, all Mills 5 watt resisitors).
Not surprising the 16 ohm tap sounded better. As for which resistance across the Paramount worked best, I think the 20R or 22R sounded best but there was not a big difference between all four. When the second channel is built and listening in stereo I may be able to refine this more.
As for my listening impressions. First it had plenty of volume, louder than I would want to listen at. The F4 adds very little to the coloration of the sound, except more power and the bass was very noticbly more present. Still warm and very detailed, much like my SET amp on steroids.
I also listened to just the preamp--->F4 combination and found this worked well also but the sound was much more alive with the SET amp in the chain.
The entire system was dead quiet no hum at all, even at loud volumes.

I look forward to trying it in stereo in my actual system (BeePre and SR45amp)


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Debra

[mcandmar]

Are those boards from the diyaudio store?  I've been tempted to build one of those amps as they seem very popular on the forum.

Love the purple wiring by the way

[debk]

Yes they are the boards from the diyaudio store

Deb

[debk]

It really is a nice sounding amp especially when driven with a DHT tube amp.

Glad you like the purple wiring, I kind of have a thing for purple 

[kgoss]

Really nice build Deb. Please let us know how it sounds once you get the other amp built.  If it really does sound like a high power 45 amp when driven by your SR45 amps then it could be the best of both worlds. A SET amp sound with enough power for any rock or classical music. Very interesting.

Ken

[bjorgens]

debK
If I may ask, what front panel switch did you use, and how did you wire it in?  I.e. are you switching the mains  directly?

bryan

[debk]

I got the switch from apexjr.com.  I am switching the mains.  There is also a mains switch on the back, but i wanted somehting up front also.  The rails in the amp are 24V, perfect for the led ring light in the switch

[Jim R.]

Hi Deb,

I'll admit to playing with some SS stuff too -- namely the Aleph J (in monoblock form) and I just couldn't pass upthe ACA amp -- another SE class A SS amp but this one with a single output device, and if you rebias it as per Nelson's instructuions, it's good for a clean 6 watts.
 I do have a pair of F4 boards here too.

-- Jim

[mcandmar]

Debra, Have you any idea what kind of voltage this amp is seeing on its input, i assume its way above line level being unity gain?

Reason i ask is i'm planning to refurb a pair of vintage speakers so i need an amp with ~25w per channel to drive them.  I'm trying to work out if an F4 with a Bottlehead Preamp in front of it would be enough to drive a set of speakers from a line level input, or would i need a amplifier in front of it for a greater voltage swing.  I have no idea what kind of voltages speakers usually run at.

Cheers,

Mark

[Paul Birkeland]

Ohms law says that 25W into 8 Ohms is about 14V.

A Crack might be a good choice to drive a unity gain amp that needs that kind of input voltage.

[mcandmar]

Thanks Paul, didn't realize it was that simple to work out.   I was thinking about the Smash kit but it seems 7vrms wont be enough in that case.   Maybe i should just build a regular F5 and use a preamp in front of it, decisions decisions.

The blurb for the speakers says the requirements are 15-30w, with 15w into 8ohms producing 96db at one meter in anechoic conditions so 25w may be complete overkill.

[Jim R.]

The F4 is a current amp only -- no voltage gain. Rule of thumb is that you need a preamp of 12-15 dB gain. That's a bit much for the bh preamps. The Aleph J is a much better amp than the F5 and is one of Nelson's personal favorites of all his designs. This is typically the amp that people who like SET/tube amps go to because of it's predominant second order distortion characteristic, and will work fine with BH preamps. I believe it is also easier to build than the F5, but I'm not sure about that.

The F4 is the amp to use if you have a SET amp and speakers that need more juice and you don't want to sacrifice your SET sonics. Also a great amp in it's own right.

-- Jim

[Paul Birkeland]

The Smash will have an upgrade package that should push the gain into that region.

-PB

[Natural Sound]

The Smash will have an upgrade package that should push the gain into that region.

-PB

Are we talking C4S upgrade?

[Paul Birkeland]

Yeah, when we wrap up shooting the manual and getting all the parts in, I will delve a bit more into what it will look like.  As the stock circuit is designed, it would benefit greatly from a C4S.

-PB