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Sherwood S4400 - preamp isolation switch?

basplin

basplin

Active Member
I've got this Sherwood S4400 - interesting amp. It's a mono amp with a stereo preamp, made so you can plug in a power amp and turn it into a stereo amp. Or you can use it as a regular mono amp as-is.

In the back of this there are two RCA 'preamp out' jacks, labelled 'Chan 2' ( for said add-on mono power amp) and 'Chan 3', which I assume is a 'folddown' of channels 1 and 2 for a 3rd speaker in the middle. What I want to do is rewire that Chan 3 RCA plug to have "Chan 1" alone out there, and THEN (if needed) wire in a switch somewhere that would allow me to divert the Channel 1 preamp from the onboard power amp to this 'RCA Out' in the back, transforming this from a mono amp to a stereo preamp at the flick of a switch. Is it possible?? Would I even need the switch to do this, and just rewire the Chan 3 output plug??

A shot of the bottom and the schematic attached to aid interested parties. Thanks in advance!

IMG_4223.jpg IMG_4224.jpg
 
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looks like there is some extra stuff around the input of the on-board amp. Looks like the easy way to get matching preamp outputs is to take an 0.047 cap from pin 6 of V5 to your other output jack. It should still work fine as a preamp output as long as you don't load it down too much. An amp with a 470k input should be fine, but I wouldn't go lower than maybe 100k. It probably won't drive it very well. Switching the internal amp out of the circuit won't really help or hurt it.

looks like the other output jack is channel 1, but taken after the output transformer and using resistors and such to knock the signal level down. I suppose the goal is to have a basically-equal level output to the channel 2 out, but it adds in all of the power amp stuff to the signal path.

just be aware you'll need to keep a dummy load or something connected to the output jacks on this otherwise you risk damaging the internal amp. Would be way cool to land the matching external power amp and use this as it was meant to be used, but those seem even more rare than the 4400 is.
 
"Chan 3" would need a connection to a second amp to be a mono center channel. As-is, it's a sample of channel 1 only, not very useful. If you remove the resistor divider and connect the RCA jack to the grid of the 7199, you could just remove the 7199 to use it as a preamp (I'd remove the 7189s as well, to extend their life...)
 
Thank you both for the replies! I should have looked at the schematic again before I posted because I had forgotten that Chan 3 is actually coming off Chan 1 and is not a fold down of both channels. I remember trying to figure this out a couple years ago....

gadget73 said:
looks like there is some extra stuff around the input of the on-board amp. Looks like the easy way to get matching preamp outputs is to take an 0.047 cap from pin 6 of V5 to your other output jack. It should still work fine as a preamp output as long as you don't load it down too much. An amp with a 470k input should be fine, but I wouldn't go lower than maybe 100k. It probably won't drive it very well. Switching the internal amp out of the circuit won't really help or hurt it.

looks like the other output jack is channel 1, but taken after the output transformer and using resistors and such to knock the signal level down. I suppose the goal is to have a basically-equal level output to the channel 2 out, but it adds in all of the power amp stuff to the signal path.

just be aware you'll need to keep a dummy load or something connected to the output jacks on this otherwise you risk damaging the internal amp. Would be way cool to land the matching external power amp and use this as it was meant to be used, but those seem even more rare than the 4400 is.
Click to expand...

Gadget - I know, having a S360 would be SO COOL. I remember seeing one or two on the auction site a long time ago and being horrified at the price. I think I remember coming to your solution a couple years ago, but I was discouraged because I thought that because V5/Pin 6 was already feeding the preamp signal to the power amp, I'd be getting a weakened signal if I also connected it to the RCA preamp output as well...I assume this is not true, ha. Just to make sure, I will be able to still use the amp as a mono amp if I do this, right? Also, do I need to add a 1.5meg resistor to ground (R91) to the new Chan 1 preamp out also, or does that not matter?

Tom Bavis said:
"Chan 3" would need a connection to a second amp to be a mono center channel. As-is, it's a sample of channel 1 only, not very useful. If you remove the resistor divider and connect the RCA jack to the grid of the 7199, you could just remove the 7199 to use it as a preamp (I'd remove the 7189s as well, to extend their life...)
Click to expand...

Tom - this is a very elegant solution, but I may also be using this as a mono amp as well, so I don't want to totally disconnect/alter the connection to the power amp from Chan 1. I don't really understand though....what was the purpose of "Chan 3" then if it was not a fold down of both? Why do you think it's wired the way it is?
 
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pulling the output tubes would mess with the voltages elsewhere, so that may not be ideal.

Shouldn't be a problem if you tap the output from V5. The internal amp loads it very lightly. The internal amp would still be intact and working fine off the preamp.

The 1.5M won't hurt. Its mostly there to deal with any tiny leakage that might get through the cap.


possible you could build your own S360 if you can find a semi-trashed 4400. Just take the output stage and power supply from the 4400 and put it on another chassis. Not precisely an S-360 but probably as close as its going to get.
 
Good call - I think all the tubes will stay in, for ease of use. I'll go ahead and make this mod as soon as I finish recapping it. Thinking about adding an Inrush Current Limiter....CL90 work alright? The 4400 is rated at 110W/.95A...

Thats a really good idea on a DIY 360....it would be easy enough to do. I should keep an eye out for more of this iron....

gadget73 said:
It should still work fine as a preamp output as long as you don't load it down too much. An amp with a 470k input should be fine, but I wouldn't go lower than maybe 100k. It probably won't drive it very well.
Click to expand...

I meant to ask before, why is this? (I know nothing about preamps) If I have a power amp with impedance less than 100K - say, 30K - is there a way to increase that number?
 
The output is off the plate of the 12ax7, which is not a particularly low impedance source. From another discussion

How To Calculate Preamp Output Impedance?

The presence control looks like it either bypasses or un-bypasses the cathode. Un-bypassed you're basically at a 47k output impedance. Bypassed its 26.8k. Fed into a low impedance source you'll likely find the preamp just doesn't have the snot to drive it and overall output ends up weak. Often the LF just goes away and it sounds overall very lacking in dynamic range.

if you stick a buffer stage between the preamp and amp, that will take care of it. Better preamps have an internal buffer so they can drive a lower load.
 
Voltages may rise if tubes are removed because of no load on that particular circuit or the entire circuit as a whole. Personally I have not found this to be too much of a problem but it might wear on other tubes left in the circuit prematurely.

Its probably not much of a problem with the heater circuit but you might see it in the high voltage circuit.
 
exactly. the power supplies are not regulated and removing the output tube load will make the voltages climb a fair bit. Mostly the HV, but the heater supply will go up some as well.
 
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