AES SET II Amplifier

zingo

I just picked up this Audio Electronic Supply SET II amplifier from the 90's, and it's an interesting little amp. It's basically a copy of Nelson Pass' Zen amplifier that was commercially sold for just a short time before the amp got pulled for obvious reasons. Being that a copy of the Zen amp, the circuit is fairly basic, although this one is slightly different than the Zen. It is a great sounding amp, but it has too much buzz in both channels, more so in the left. I'm hoping a simply recap of the power supply filter, and doubling of the capacitance should take care of the issue. I also noticed last night that there is intermittent snapping in the left channel, which seems to come and go. Any idea of what that is?



Thanks for all the help, and I'll keep this thread updated as I work through the minor issues on this great amp!

zingo

Why are screw-top "blue" caps so much more expensive than equal value snap-in of the same manufacturer? On Mouser, the screw-top version of a .022F 50v is $25 verses $7 for the same cap in snap-in version. Any one have an idea on this?

megasat16

Screw tops are usually and traditionally more expensive than the snap in or solder type. I guess it's coz of the demand for screw types is less than the snap in or solder type. So, the screw types are produced less and cost more just like everything else.

And some screw types made for Audio applications (lower ESR, better construction, longer life expectancy, higher ripple rejection, etc.) usually costs a lot more than the general purpose caps. Having said that, I think the audio grade axial type and snap in / solder type caps may cost as much as the audio type screw caps.

megasat16

zingo wrote: »

It is a great sounding amp, but it has too much buzz in both channels, more so in the left. I'm hoping a simply recap of the power supply filter, and doubling of the capacitance should take care of the issue. I also noticed last night that there is intermittent snapping in the left channel, which seems to come and go. Any idea of what that is?



Thanks for all the help, and I'll keep this thread updated as I work through the minor issues on this great amp!

hmm...buzz? At around what frequencies? recap is always a good idea but don't double the capacitance unless you are sure the small transformers and the rectifiers are up to it. But about as much as 50% increase in filter caps may be fine in most cases.

I saw one small electrolytic in each channel near the heatsinks. I don't know where or what they are used for but can't go wrong to replace with a good one.

Dry out caps in the signal lines may cause drop outs and intermittent problems and occasional snaps.

FTGV

After looking at that shumozzle I'm not so sure your buzz isn't from a poor grounding aranegment instead of aged 'lytic's.Typically there should be a star ground where all grounds interconnect at one point only but I can't see if thats the case with that rats nest.Aswell it appears they have the coils in there for the PI filter as per the upgraded ZEN.

zingo

I hope you're not right about the grounds Fred, but I'll be testing the caps soon which should give me some good information. You are correct that the inductors are on each side between the caps already, so as long as the caps are bad, it should be an easy fix.

I though you'd find it interesting Fred that instead of the pair of 2200uf caps per channel, the SET II uses a signel 4700uf bypassed by a 51uf AEON poly cap.v It's a strangly designed amp and is not an exact copy of the Zen, but sounds pretty good none the less.

There is a pair of AEON in the amp that is on the signal input (51uf I think), so those may benefit from a replace as well; which will hopefully fix the snapping as posted above. I've never heard snapping in an amp channel before, but it would be great if it was bad caps.

FTGV

First sorry if my previous comments in regards to it's contruction were a bit too harsh.Since these were available as a kit,,it appears this may have been one and not factory built.Otherwise one would expect a tidier job of wire management.

zingo wrote: »

I hope you're not right about the grounds Fred,

It's a possibility after getting a look at the inards. If it's a 60hz hum then it's a certainty.

so as long as the caps are bad, it should be an easy fix.

Yes it would be,what is the value of the four main filter caps?

I thought you'd find it interesting Fred that instead of the pair of 2200uf caps per channel, the SET II uses a signel 4700uf bypassed by a 51uf AEON poly cap.v It's a strangly designed amp and is not an exact copy of the Zen, but sounds pretty good none the less.

Well by using the single 4700uf vs a pair of paralleled 2200uf's it simplifies and reduces cost.Since these electrolytics are used for output coupling,bypassing with a large film is ceraintly a good idea.I did this as well but used a much smaller film cap.

There is a pair of AEON in the amp that is on the signal input (51uf I think), so those may benefit from a replace as well;which will hopefully fix the snapping as posted above.

The stock Zen used a 47uf 'lytic' at the input so the 51uf of film is prefered.I doubt the cap is faulty so it wouldn't be high on my list of things to replace.For the source of snapping I would first check all solder joints and connectors such as the ground lugs on the RCA input's etc.

zingo

The filter caps are 10000uf each as per the origional specs.

zingo

I went through the amp and unscrewed and retightened the leads on all the caps, and double checked connections and solder joints. This seems to have reduced the buzz a little and eliminated the snapping, but I'll be taking an LCR meter and oscilloscope to the amp and caps to see what's going on; and before I make any purchases.

I got the chance to sit back and take in a few albums tonight and the amp rocks. It's a great combination with the JFET based Forte F44 preamp and custom Klipsch Forte speakers; plenty of gain and not a hint of SS grain or harshness. I'll be bench testing the amp soon enough, but it's still a very enjoyable listening experience as is! :biggrin:

FTGV

zingo wrote: »

I went through the amp and unscrewed and retightened the leads on all the caps, and double checked connections and solder joints. This seems to have reduced the buzz a little and eliminated the snapping,

Kewl.

It's a great combination with the JFET based Forte F44 preamp and custom Klipsch Forte speakers; plenty of gain and not a hint of SS grain or harshness. I'll be bench testing the amp soon enough, but it's still a very enjoyable listening experience as is! :biggrin:

Have you tried lifting the chassis ground on the amp to verify that the problem is'nt a system ground loop instead of an internal issue with the amp?Unfortunately with the high sensitivity of your speakers any residual noise will be more apparent.

btw.do you happen to have a schematic for that Forte?I would be interested to see how it's jfet gain stage compares to what I'm using.

zingo

I use PS Audio power cables that have a removable ground pin, so I popped out the pin for the amp, plugged it in, and the buzz/noise level didn't change. So I guess that is good and bad news at the same time.

Also, I noticed there are little blue pots mounted next to each of the large, black caps on each wall of the amp. Those would be a bias pot if I'm not mistaken? I'm not aware that Nelson included any bias circuits in his designs, but this amp seems to be a mix of good and bad ideas...

FTGV

The bias is non adjustable,the pot is there to allow adjustment of the DC operating point. However unless you have a signal generator and can monitor the output waveform on a scope it's best to leave that little blue pot alone.If you have the scope and generator you would apply enough signal to just drive it to the point of clipping.If the observed waveform is clipped more at the top or bottom you would adjust the pot to make them symmetrical.

zingo

Thanks and I'll look at those pots next time I have the oscilloscope and signal generator out.

After two days of listening snap free, the snapping noise came back in the left channel during one of my two listening sessions last night...

heiney9

I'm with Fred, make sure there is a competent ground scheme throughout the entire amp before checking anything else. Many times some DIYers forget or just don't know how important proper grounding throughout the entire circuit is.

H9

zingo

I'll need to sit down one night and go through the amp with a fine-tooth comb and check every connection and solder joint; including making sure there are appropriate grounds in the circuit. The wiring is a little nuts, but it's also a great sounding amp, so I don't want to make too radical of changes while address these few issues.

heiney9

Configuring a "Star" ground, if there isn't one, is the best way to go, and it could improve the sound not degrade it. I've just seen it before on a lot of DIY builds, the ground plane is the most common issue and it's easily remedied.

Good luck Jake

H9

zingo

heiney9 wrote: »

Configuring a "Star" ground, if there isn't one, is the best way to go, and it could improve the sound not degrade it. I've just seen it before on a lot of DIY builds, the ground plane is the most common issue and it's easily remedied.

Good luck Jake

H9

Like this?

One of the best amplifier power supply grounding schemes is a "star" ground system, where all the local grounds for each stage are connected together, and a wire is run from that point to a single ground point on the chassis, back at the power supply ground. Even better is a two-point star, where the power supply grounds (PT center tap, first filter cap ground) and output stage grounds (output tube cathodes for fixed bias, or cathode resistors for cathode biased, and output transformer secondary ground) are connected together and to the chassis at a single point, right at the ground of the first filter capacitor. The ground of the second filter capacitor, after the choke or filter resistor, is the star ground point for the preamp stage grounds. Use a local common point for each preamp stage ground, and run a wire from this common point back to the second star point. If two stages are out of phase with each other, the can share a common local ground, but don't use more than two stages per local common ground. This concept can even be taken further, with multiple star points for various amplifier stages.

FTGV

zingo wrote: »

Like this?

One of the best amplifier power supply grounding schemes is a "star" ground system, where all the local grounds for each stage are connected together, and a wire is run from that point to a single ground point on the chassis, back at the power supply ground. Even better is a two-point star, where the power supply grounds (PT center tap, first filter cap ground) and output stage grounds (output tube cathodes for fixed bias, or cathode resistors for cathode biased, and output transformer secondary ground) are connected together and to the chassis at a single point, right at the ground of the first filter capacitor. The ground of the second filter capacitor, after the choke or filter resistor, is the star ground point for the preamp stage grounds. Use a local common point for each preamp stage ground, and run a wire from this common point back to the second star point. If two stages are out of phase with each other, the can share a common local ground, but don't use more than two stages per local common ground. This concept can even be taken further, with multiple star points for various amplifier stages.

Yes thats what I was alluding to in post#5.Not having a star arangement where all gorund currents are retrurned to single point can lead to hum issues.And yes redouble check the integrity of the solder connections and those locking nuts on the RCA ground lugs etc.
Also do you have any caps of reasonable size and appropriate voltage that you can quickly parallel in the circuit to see if that mitigates the hum to any degree?

zingo

I checked the amp this morning, and all grounds ran to a lug next to the IEC socket, which was then wired to the ground pin on the IEC socket. Does this sound like a good star configuration? I will be using some leads to add new grounds to different points in the circuit and see if anything is not currently properly grounded.

On the same topic, the ground on the input RCA sockets should be grounded to the star, but isolated from the chassis, correct?

FTGV

zingo wrote: »

I checked the amp this morning, and all grounds ran to a lug next to the IEC socket, which was then wired to the ground pin on the IEC socket. Does this sound like a good star configuration?

That should be fine .

I will be using some leads to add new grounds to different points in the circuit and see if anything is not currently properly grounded.

You have to be careful as adding extra grounds may create a loop.

On the same topic, the ground on the input RCA sockets should be grounded to the star, but isolated from the chassis, correct?

Correct for the same reason mentioned above.

zingo

Update: The amps doing very well with a little adjusting of wiring. I'm amazed by how good it sounds for such a simple design, and this amp is the best sounding SS amp I've had the pleasure to own. I'm still adjusting the grounds to get the noise even lower, but it's a great amp for any type of listening. Next I'll be putting it head-to-head with a KT88 tube amp to see the comparison.

I saw this idea to make a cap discharger which I'll being doing shortly with a 10K resistor and a popsicle stick; to avoid the insulated screwdriver approach...