Sherwood AM-8500B MOSFET power amp

[helma]

Went to check out a local thrift store and found this one for a price I just couldn't refuse. Didn't know a thing about it, but since it was a heavy beast advertising 200W / channel @ 8 ohms and I could see two quite beefy transformers inside, I took it home. Found a service manual online, and the first thing I notice is there's an op-amp right at the input, that was a bit discouraging. Not that I'm against op-amps or that they can't be part of a good sounding design, just that often that's not the case. Looking further there's a FET input stage and MOSFET powerstage with 4 SK1058s & 2SJ162s per channel.

Well what do you know, it powers on and DC-offset is very stable, both channels in the 0-2mV range so I hook it up and messing with the cables nearly blew off my ears and speakers because I forgot to turn off the speaker outputs. I usually use cheap and disposable speakers trying out gear in unknown condition, but perhaps luckily went for the JBLs this time because anything else I have probably would've fried.

The only preamp I own at the moment is a cheap Little Bear that's been sitting on the shelf gathering dust. It's not a great piece of kit, but it's decent enough. The Sherwood gives a pretty good first impression, definitely has some serious grunt and power available. A bit forward in the midrange but it could be the pre, it's been a while since I used it and I think it might've been a bit midrangey sounding with the valves I have in it currently. I'm not a big fan of MOSFETs in general but I'd say this one sounds better than the Adcom something or other I had (I think 5300).

So now I have 25kgs of power amp I have no idea what to do with... at least I got it cheap A little picture straight from the top:



and a link to google image search, which brings up some nudies among the first results: https://www.google.fi/search?q=sherw...dPKOwsjVp_XVM:

[walpurgis]

That looks interesting Kai. It's not an amp I know. Fine display of heatsinks inside!

I've got nothing against the sound of MOSFET's in general. They can sound good.

[helma]

Yeah I think Sherwood was more famous (or not) for budget gear, this one seems a more serious effort though. I've been listening to it for a few hours now, it's a good sounding amp with very healthy power reserves and good dynamics.

The thing with MOSFETs is ever since I got extensive experience with a MOSFET based guitar amp (there was a time in the 80s MOSFETs were all the rage for trying to get a valve like sound and distortion characteristics/behaviour from a transistor amp), there's a certain "MOSFET tinge" in the sound I seem to often hear. Not so much with this one as the Adcom though, actually I'm not 100% I'd notice it in this Sherwood if I didn't know it was a MOSFET design, where as with the Adcom it was very clear - I had no preconceived notions of how MOSFETs are supposed to sound when I fired that one up, but it instantly brought back memories of that Marshall Lead 20 or whatever it might've been called.

Guitar amps really pronounce differences since things are often driven to distortion - even "clean" tones are often on the sweet spot right at the edge of break up. I used to think my experience with guitar amps was good for nothing regarding hifi, thinking they are completely separate worlds since one is about just producing what is there cleanly and other is about making the amplifier an instrument in itself. But I've come to find out the sound characteristics tend to be there in hifi as well (valves vs solid state, bjt vs fet etc.) just subtly in good kit at least.

In any case I'd love to hear this one with a more competent preamp. Maybe I'll try putting together a little passive volume control from surplus parts (if I have some). Need to give this one some time (and time for my ears to adjust), might end up being a keeper.

[helma]

Well, I swapped the valves on the Little Bear for JAN5654 since I recalled liking them better than the Mullard EF95s which were there now. I recall only ditching the 5654s originally because one of them was suffering from microphonics and nasty noises whenever the fridgerator compressor switched on. No nasty noises this far though. I don't remember there being much between them in terms of sound the last time I swapped, but with the Sherwood I like the 5654 a lot better. The midrange especially seems now more open and less harsh. It also made me remember why I used to like this little pre. I think it's a transistor gain stage followed by a tube buffer but haven't bothered to try dig up a schematic, certainly nothing special. In any case it's not bad at all, especially considering I think I paid about 20 euros for mine + shipping... and with the JAN5654s it makes very sweet music with the Sherwood. I'll give this combo some time to get used to it and then compare against the Pye HF10 stereo pair and a Yamaha A-700. The Yamaha btw. is a fabulous amp for the money they go for, but they some caps which are almost always way past their best-before date at this point, so they do need a recap.

[Arkless Electronics]

You didn't say how cheap you got it for but it looks like a lot of amp.... Those mosfets are about £10 each for a start! I like mosfets personally BTW...

[helma]

You didn't say how cheap you got it for but it looks like a lot of amp.... Those mosfets are about £10 each for a start! I like mosfets personally BTW...

The shop is closing and everything was on sale, so it had a sticker of 55 euros on it. I agreed to pay that and went to get my car so I wouldn't have to carry the thing around, when I came back and was handing him the money he said 40 is enough. I didn't complain... The amp had been sitting in the corner of that shop for quite some time, I think 2 years or so. It had caught my eye before, but their original asking price was 250 eur if I recall correctly, in any case too much for me to take a punt, but now having this thing home there's no way I would let it go for much less than that. I did some further digging, it seems to be pretty much identical inside to model AM-7040 and built in Korea by Inkel, which still owns the Sherwood brand.

Some specs:
Output power 20Hz-20kHz - 200W @ 8ohms (<0.04% THD), 320W @*4ohms (<0.08% THD). There's no rating for 2 ohms apart from dynamic power, which is specified at 650W (and 450W and 250W for 4 & 8 ohms). At -6dB output THD is supposedly < 0.01%. Damping factor 150, freq response (-3dB) 5Hz-150kHz. Signal to noise 120dB / 90dB (unweighted). All in all seems pretty healthy and a lot better than the Adcom I had, and my ears agree.

Still wanna hear this with a better pre, I don't think the Little Bear is capable of letting the amp show it's true color, or perhaps lack of Also need to dig up the Kef 104aBs at some point, they reveal certain things better than the JBLs. But the JBLs (120ti) stay so clean at high volumes and having 200W at hand to drive them is addicting.

[Macca]

Yeah I think Sherwood was more famous (or not) for budget gear, this one seems a more serious effort though. .

In the UK yes but in the States they are quite well regarded at the NAD/Rotel sort of level. I think the UK reputation stems from when Richer Sounds used to sell a very low budget line of Sherwood that was less than stellar in build and reliability, but they were very cheap. I think they did a receiver for £49.99 and a cd player at £39.99. One of my brothers had the reciever. It was okay.

[Arkless Electronics]

The shop is closing and everything was on sale, so it had a sticker of 55 euros on it. I agreed to pay that and went to get my car so I wouldn't have to carry the thing around, when I came back and was handing him the money he said 40 is enough. I didn't complain... The amp had been sitting in the corner of that shop for quite some time, I think 2 years or so. It had caught my eye before, but their original asking price was 250 eur if I recall correctly, in any case too much for me to take a punt, but now having this thing home there's no way I would let it go for much less than that. I did some further digging, it seems to be pretty much identical inside to model AM-7040 and built in Korea by Inkel, which still owns the Sherwood brand.

Some specs:
Output power 20Hz-20kHz - 200W @ 8ohms (<0.04% THD), 320W @*4ohms (<0.08% THD). There's no rating for 2 ohms apart from dynamic power, which is specified at 650W (and 450W and 250W for 4 & 8 ohms). At -6dB output THD is supposedly < 0.01%. Damping factor 150, freq response (-3dB) 5Hz-150kHz. Signal to noise 120dB / 90dB (unweighted). All in all seems pretty healthy and a lot better than the Adcom I had, and my ears agree.

Still wanna hear this with a better pre, I don't think the Little Bear is capable of letting the amp show it's true color, or perhaps lack of Also need to dig up the Kef 104aBs at some point, they reveal certain things better than the JBLs. But the JBLs (120ti) stay so clean at high volumes and having 200W at hand to drive them is addicting.

A bargain! Distortion figures seem rather high for a mosfet amp but still low enough (loads of NFB can be applied safely to mosfets and so very low distortion can be achieved... Whether or not this is a good idea is a big subject in itself!). I would not expect this amp to be capable of driving 2R loads. That would need twice as many mosfets again!

[Arkless Electronics]

Out of curiosity I had a look at the circuit.... pretty impressive in many ways! They haven't used "The Hitachi Circuit" as so many mosfet amps do but have designed their own circuitry which is completely symmetrical and appears to be a low-ish feedback design. In many respects I could have been looking at the circuitry of a £3K power amp! It is rather similar to the Lang 20W class A design but with a few further enhancements. They do rather er on the side of caution in many ways though... Preventing even a one in a million chance of damage to either itself or the speakers, even if misused, has ultimately been given priority over sound quality I reckon...
Some of the frequency compensation etc measures look a little heavy handed and brute force but without a full analysis it's dangerous to speculate too much on that... They ain't bothered by how many electrolytics are in it neither! The op amp at the input is just a buffer and shouldn't get in the way too much. It could be bypassed altogether but I wouldn't risk it unless if it is to be used with a low output impedance SS active pre only... which would probably be full of similar op amps!
Protection circuitry is comprehensive.

Bargain!!

[helma]

Out of curiosity I had a look at the circuit.... pretty impressive in many ways! They haven't used "The Hitachi Circuit" as so many mosfet amps do but have designed their own circuitry which is completely symmetrical and appears to be a low-ish feedback design. In many respects I could have been looking at the circuitry of a £3K power amp! It is rather similar to the Lang 20W class A design but with a few further enhancements. They do rather er on the side of caution in many ways though... Preventing even a one in a million chance of damage to either itself or the speakers, even if misused, has ultimately been given priority over sound quality I reckon...
Some of the frequency compensation etc measures look a little heavy handed and brute force but without a full analysis it's dangerous to speculate too much on that... They ain't bothered by how many electrolytics are in it neither! The op amp at the input is just a buffer and shouldn't get in the way too much. It could be bypassed altogether but I wouldn't risk it unless if it is to be used with a low output impedance SS active pre only... which would probably be full of similar op amps!
Protection circuitry is comprehensive.

Bargain!!

Thanks, I had a look at the schematic myself, but beyond "oh that looks kinda impressive" it was way over my head, so nice to hear from someone who knows their stuff. I'm not too worried about the op-amp and won't bother messing with it and it seems they used a good one (NJM2068). I know it's an old one and probably there are better ones available these days, but I have some experience with it in phono preamps and I like how it sounds.

I haven't even took a look under the hood yet, but if it looks like it won't be too much of a pain to work on I'll probably do a recap over the winter and substitute with film where sensible. I guess this was made in the late 80s or early 90s probably, so it might be the caps are still mostly fine - or not. I recapped both a Yamaha A-700 and A-520 last winter - caps in the latter were mostly just fine, despite the caps being of cheaper variety and rated tighter, while the A-700 had all fancy Nichicon muses, but most of the 200-1000uF caps were like half of their original value if that and one 680uF was just totally gone. Maybe Nichicon just happened to have a bad run back then, it seems a common fault in the A-700 and there was also the famous glue that turns into something that eats through metal, so maybe that also has something to do with it.

I'll keep the Sherwood in the system for some time, nothing I don't like about it yet, it might just be the best solid state piece I've had in the house but I only got it yesterday, so it's early days. Also the JBLs are not the best speakers for holographic sound staging and what not, they paint with a rather broad brush no matter what the amp, and while the titanium tweeters are very revealing of any grime in the treble and generally very detailed (more so than the Kef T27), still the Kef 104aBs I feel show a deeper picture into what's going on earlier in the chain as a whole. Not sure how they do it to be honest.