2sk170 phono stage on PCB

[jsn]

http://boozhoundlabs.com

Sounds pretty great, and the PCB was almost right :)

jsn

[Matt Wiebe]

Jason,
Totally cool, love the simple layout. I've been wanting to knock off
a board design for the Radio Shack phono amp because they are so
simple. So are you stepping up the Denon with a SUT into they Jfet
board?
Matt

[jsn]

Yep. What's the Radio Shack phono?

jsn

[jsn]

For stepup, right now I'm using an Altec 15095A. I have a 20k
reistsor across the secondary, which, when paralleled with the 47k
input impedance of the phono, give you about 15k. This reflects 150
ohms to the denon. I played with loading using a pot, and it appears
that the sound gets better and better (less bright, and better bass)
until you get to about 100 ohms, where it sort of falls apart.

I have a 1:10 cinemag on order. That should sound really good I
think. 1:10 is just about right for the denon I think, since that
gives you 3mV which is in the range of what most phono stages expect.

jsn

[Matt Wiebe]

Jason,
The Rat Shack Phono (Model #42-28250) was a small phone preamp that
ran off a 9V battery. The entire package was the size of a pack of
cards. Very nice sounding unit that runs about $12-$16 on Ebay. I will
try to drop in a schematic below if that works.

[jsn]

Oh yeah - I have one of those :)

jsn

> I also ended up with 100 ohm load for my Denon, they do sound very nice.
> Matt

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[rjgu...@gmail.com]

Hey guys,

In my quest to make the ultimate small amp, I am taking some serious
journeys into big amp technology.

Would you mind looking at this schematic and let me know if you think this
amp would gain any advantage in staying super clean by using a cathode
driver in front of the output tubes as opposed to no driver at all?

I haven't made any real effort to get all the resistors and coupling cap
values right but would appreciate your input on those items as well.

thanks very much,

rjh

[jsn]

The only showstopper is that the grids of the driver tubes are not
referenced to ground. There are a couple resistors that seem too big
to me too, but that's no big deal. I'll stop by today.

jsn

[Matt Wiebe]

It's a little hard to know what you want. The cathode drive will give
you a low impedance driver, but the coupling cap still keeps DC off
the grids. Since DC is blocked as the grid moves close to 0Vs its
going to clip like usual, low impedance driver or not. If you have
power tubes that can handle grid current and direct couple your
cathode drive to the power outputs, then you can drive it cleaner I
expect. You've plenty of power here, you can also keep distortion low
by running the grids deep Class A and keep the coupling caps in. But
again I'm not really sure what you mean by super clean, but DC would
be lower distortion.
Matt

> <SSS Power Amp - Oct 13, 2009.jpg>

[jsn]

Bob, can you post the original schematic this is based on? I can't
remember if the drivers were DC coupled to the output grids or not,
but it is possible they were.

Jason

[rjgu...@gmail.com]

Jason,

No problem... I've got the original AMPEG and also the exert I made from
it... I'll be very curious how far or how close to the original AMPEG i end
up.

Matt, the idea was that this AMPEG bass amp was an immensely powerful amp
that never went into distortion when used as a guitar amp. One key is said
to be the cathode driver into the power amp tubes... I would like to take
advantage of those things that make the amp clean but also want to simplify
the crcuit if it doesn't change the outcome.

thanks,

rjh

[Matt Wiebe]

An advantage of being all powerful is you rarely clip. So the short
answer as to why the Ampeg didn't clip is it had more power than was
needed, not its topology.
Making a lower power amp keep from clipping is harder but the things
I pointed out in my prior post still do that. If your power finals are
not up to the job of the loudness you seek out of the speaker no
topology will save you. Having more power then the speaker needs will
always keep you in the low-distortion area.
Low-impedance drivers have more impact on bandwidth than anything
else, that is apart from adding an additional stage.
Matt

> <svt6550-jp.gif><SSS - Ampeg SVTdriver and power tube circuit.jpg>