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:

http://i.imgur.com/yfhhhpV.jpg

and a link to google image search, which brings up some nudies among the first results: https://www.google.fi/search?q=sherwood+am-8500&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiJ7PbyvY7WAhWjNJoKHdlSDTQQ_AUICigB&biw=1200&bih=1160#imgrc=QdPKOwsjVp_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.

[helma]

Well I had a bit more of a look around the circuit and upgrading some of the caps seems like a no brainer to me. For one thing right at the output of that buffer opamp there's a 4.7uF electrolytic cap which is not even bipolar(??) - then at the input of the following gain stage there is a 5uF bipolar electrolytic (actually two 10uF polar caps back to back). Replacing these with 4.7uF film should yield SQ benefits I think. Or perhaps this would be an opportunity to mess around with the Russian PIO caps I've been curious about, if they fit. Then at the output of the preamp stage there are 0.047uF ceramics, again probably better replaced with film I think. I'm not hell bent on replacing all the caps, but beyond the ones I mentioned I think it's probably a safe bet not all the electrolytics in the several hundred uF range are no longer up to spec. The smaller ones I guess might be fine, at least in my admittedly limited experience the small ones usually hold up better.

[tallauscher_]

Hello,
I just read your report with great interest.
You realize that you are writing here about one of the best kept secrets in the HiFi world?;)
I myself own 2 of these amplifiers and thus operate very "critical" speakers in bi-amping. The impedance of these speakers goes up to 1ohm. The power amplifiers do it with pleasure.:smoking:
Have fun with your power amplifier, it's best to close this report quickly so that no one learns.:D
greetings from Germany

[tallauscher_]

Good evening Kai,
somebody in the USA did a nice new comment for the "powerhouse", please read:
Alright....so it's 7 years later from the last post. No biggy. The amp is the biggy. I almost needed another can of spinach to help me carry the Sherwood to and from my car into my home. Then find a place/ shelf strong enough to support the weight and look good doing it. Success.
After hooking it up to main out jacks of my Concept 4.5D (connected to a Dual 830c cassette deck and a Technics
SL-1200mk2, later a CD player ). The Sherwood amp came with a adapter so it will fit in USA electrical sockets. The original plug prongs are round, Singapore perhaps? Defiantly not made for America to enjoy. Inkel has the whole matching set.....AVP8500 preamp, cassette deck, equalizer, compact disc player, and tuner. Speakers too. It's a beautiful set.
Anyway this monster gold brick of an amp is mighty powerful and well built. Supposedly has many high end parts. This is my first amplifier. I'm still learning. Internet research on these turns up little results and information. That's makes this a rare treat to own. And a treat it is to listen to. I've owned it for over a year and also picked up some other brands amps which I've compared them all. The Sherwood wins! Clean power, wide frequency range, high damping factor, and A/B speaker selector. Sound transmission is straight. To me no colorization or brand traits come through, unlike Denon or Sony. Another nice thing is it doesn't run HOT! It's worked flawless since I got it.
Overall I think this amp is great. I have nothing bad to say about it.

The 8500 is nearly the same like the older AM-7040, the older one is slightly better. If you want i can demonstrat picutes here. Interesting is one of the last sentences about running hot.
I owned 2 Krell KSA 150 - really "Monsters" and legends, but they did not do a better job at my speaker misfits..... But when the Krell's were running, it got very, very hot in my music room:cool:
Greetings
Axel

[helma]

Hello,
I just read your report with great interest.
You realize that you are writing here about one of the best kept secrets in the HiFi world?;)
I myself own 2 of these amplifiers and thus operate very "critical" speakers in bi-amping. The impedance of these speakers goes up to 1ohm. The power amplifiers do it with pleasure.:smoking:
Have fun with your power amplifier, it's best to close this report quickly so that no one learns.:D
greetings from Germany

Hi Axel,

Yes I've been quite impressed with this beast. I didn't notice your posts earlier, but since I just made a little DIY passive "pre-amp" I was about to update this thread. Up until now I've been using the Sherwood with the cheap little chinese tube preamp, today was really the first time I got to hear the amp properly on its own without the Little Bear pre coloring the sound. It's a nice enough thing and better than you'd expect for the 20 or so euros they sell for, especially after changing the valves to something nicer, but it certainly imparts it's own sonic signature.

Now the passive I just put together is nothing more than 20k volume pot fastened into a piece of wood, wired directly into some poor quality 80s interconnects - I salvaged the pot and the RCA-cables from a cheap and defective 80s Pioneer tape deck. Bass was just fine before, but certainly has even more oomph now at the very bottom and also a tad more control. Also dynamics seem better than before, one unfortunate side effect of that is playing vinyl some clicks and pops really pop out, which probably means the amp doesn't shy away from fast transients and voltage swings :)

Also at higher volumes the sound stays better controlled, not that it was bad before but I think at higher volumes THD from the Little Bear was becoming noticeable before. Between the JBLs and the Sherwood there's a crazy amount of clean power on tap, the limit really is how much my room can take before windows and everything start to rattle.

This was just a quick try to see if a passive might be a way to go and I think I'm gonna build a proper one using some nice (hopefully...) and cheap chinese stepped attenuators.

[helma]

After my experience modding a cheap TC-750 phono pre and hearing the difference coupling caps make, I finally got around to changing the coupling caps in the Sherwood. So electrolytics out and Audyn Q4 PP film caps in. Pretty huge difference in clarity and detail. The amp sounds a bit less warm now but not at all in a bad way, no signs of stridency or coldness. It's just more like a blanket was lifted. In fact my cheap CD-player got more listenable for the change, despite the amp in general being more revealing of source material now. Spinning some discs, both vinyl and CD I first got concerned I made a bad solder joint or something because I was hearing some distortion I hadn't noticed before, but in all cases it ended up being about the source. One CD it seemed like on one track something on the right channel had been very slightly clipping during recording. In other cases it ended up being things like snare drum rattling in the background or similar fine details deep in the mix I hadn't noticed before. Part of it was also probably I was pretty neurotic in listening trying to spot distortions. Anyway, if anything the highs are smoother now. Mids are way more transparent. The only part where I'm not 100% sure all change was positive is the bass. While I feel like qualities of bass drum for example come through better now, sometimes I feel the bass got a bit overly round. But it also depends on the recording, since for the most part the change is not that noticeable with electronic music with really tight kick sounds etc. Anyway on the whole, the film caps certainly impose far less of a signature on the sound. Changing them was easy enough and didn't have to desolder anything apart from the replaced components. Couldn't fit the large Audyns directly on the PCB though, but that was no surprise and there was plenty of room to keep them close and avoid long runs of wire.

I'm pretty stoked and so glad I bought this amp. Looking back it would've been bit of a bargain even at the original 200 euros asking price.

[ReggieB]

Nice update!

[paulf-2007]

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.
Early solid state guitar amps put me off solid state for many years, they just sounded dead, no sparkle.

[helma]

Early solid state guitar amps put me off solid state for many years, they just sounded dead, no sparkle.

I was 14 when I got to play a tube amp for the first time. It was only some old 60s thing though, Vox AC30 I believe it was called ;) It was amazing, how alive it not only sounded but felt, like a huffing beast at your finger tips. And that was some damn loud 30 watts. Not understanding anything about the technical side, power ratings or speaker sensitivity, it led to the obvious conclusion "tube watts" are way more powerful than "solid state watts" :D There was really no going back to solid state after that. For some odd reason I always thought valves make no sense whatsoever for hifi. Not sure where I got that idea from, but it didn't really occur to me to challenge that notion until quite recently.

Anyway the Sherwood healed my Adcom reinforced MOSFET traumas of youth, apparently they can sound quite fine. Btw. I took a couple of photos of the insides while I was changing out the coupling caps, should post those later.