Peavey KB100 Sounds Fuzzy

[muelfo]

I was given a Peavey KB100 because the owner didn't care to have it fixed. I'm a hobby level electronics and music enthusiast. I could use this amp if I could make it work right.

The problem is the amp sounds a little fuzzy when notes are played. I'm a keyboard player. I use a Yamaha P-85 keyboard. It sounds great when played through my stereo system. But when I play it through the KB100 it sounds like the signal is passing through a low level fuzz box. In other words, there's some distortion when notes are played. Otherwise the amp is silent (no hum).

When I power up the KB100 is takes a minute or so for the volume to come up and level out. It seems like the fuzziness is less noticeable while the volume is coming up.

I measured the voltages across the power supply filter capacitors. Where the diagram says 43 volts I'm getting 41.8 volts. Where the diagram specifies 15 volts I get around 15.5. The 15 volt supply takes a few minutes to stabilize. When the amp is first switched on it starts around 14.9 volts. The 43 volt supply goes the other way. It starts around 42.5 and settles down to 41.8. The + and - supply voltages are within less than 0.5 volts of each other. The same voltage positions measured on the AC scale are about 0.1 volts on the 43 volt supply and .003 on the 15 volt supply.

I'm not sure I'm connecting my keyboard (Yamaha P85) to the KB100 properly. I'm plugging the P85 headphone output into the KB100 power amp input. This, of course, bypasses the preamp in the KB100, which bypasses the volume/tone controls. I assume this is the correct connection since the P85 headphone output is preamplified already. Also, things don't go well at all when I connect the P85 to the normal KB100 inputs.

I purchased capacitors to replace those in the power supply, but I'm not really sure they need to be replaced. As I said above the voltages seem reasonable, at least to my naive mind.

I also purchased some op-amps (4558). I replaced U1 and U7 just because it's easy to do. The high frequency notes seem a little brighter but the fuzziness is still there.

Since the amp is old (vintage 1980's?) I'm debating whether I should go ahead and replace all the electrolytics. Would that be a good idea? However I'd rather deal with the problem directly and not replace parts unnecessarily.

In summary, I'm hopeful someone has seen this issue before and can advise me what steps to take.

A wiring diagram is attached. I got if off of this form (thank you very much). It looks correct for my KB100.

Thank you for your time and consideration.

[Enzo]

I don't find a picture of the rear of your keyboard, but I have to assume it has line out jacks? headphones out is not a good place to take a signal for an amp. You could plug a line out from the keys to the power amp in, but it should also work plugging into a KB100 input jack, after all that is what the KB series was made to do.

The 43v or whatever is approximate and if it is off a couple volts, that is normal. The actual voltage is less important than that the positive and negatives are at about the same level. Also 15v supplies, again a half a volt either way is no big deal, but we prefer the two polarities be at about the same voltage.

Connect a CD player to the amp, does that sound OK, or is that fuzzy too? Or if you have an electric guitar, plug that into the KB100, still fuzzy?

[Leo_Gnardo]

I was given a Peavey KB100 because the owner didn't care to have it fixed.- - - -

I'm not sure I'm connecting my keyboard (Yamaha P85) to the KB100 properly. I'm plugging the P85 headphone output into the KB100 power amp input. - - -

However I'd rather deal with the problem directly and not replace parts unnecessarily.

Whenever I hear this kind of problem, I make it a point to check the speaker first. Sometimes it's just plain shot, or has a tear in the surround, or something's gotten stuck between the frame & cone. This last one's not to be neglected - once I saved a guitarist's speaker from an unnecessary recone - there was a dusty old potato chip jammed in there.

About using the headphone out - it's possible you're overloading the miniature amp in the keyboard that drives a stereo headphone, by inserting a mono plug into the jack and shorting out one side of that amp. That could explain a fuzzy tone. An adapter cable that has a stereo plug on the keyboard end, and simply sends one side or the other to a mono plug on the other end might help. No need to connect the unused side of the stereo line to anything.

Never a good idea to just throw parts into a unit in hopes that replacing one might be the fix. Enzo has already sent you in a good direction. I mentioned the two things I did because they also should not be neglected. It would be awful to go through a lengthy process, then eventually find the speaker was kaput, or the method of using headphone output was the cause of distress.

[muelfo]

I don't find a picture of the rear of your keyboard, but I have to assume it has line out jacks? headphones out is not a good place to take a signal for an amp. You could plug a line out from the keys to the power amp in, but it should also work plugging into a KB100 input jack, after all that is what the KB series was made to do.

The 43v or whatever is approximate and if it is off a couple volts, that is normal. The actual voltage is less important than that the positive and negatives are at about the same level. Also 15v supplies, again a half a volt either way is no big deal, but we prefer the two polarities be at about the same voltage.

Connect a CD player to the amp, does that sound OK, or is that fuzzy too? Or if you have an electric guitar, plug that into the KB100, still fuzzy?

Thank you for quick reply Enzo.

I too was surprised to find that my Yamaha "Professional" keyboard has no outputs other than headphones and midi. I also wondered what kind of keyboards this KB100 "keyboard amplifier" is made for. Must be for keyboards that have no internal amplifier, evidently.

I plugged in an electric guitar (circa 1965 Fender Duo Sonic). It sounds fine. The amp has more continuous hum when the guitar is plugged in, but I've heard much worse. I do notice that after a string is plucked and the sound is fading down to almost nothing there is a faint fuzzy sound that becomes more obvious as the string vibration fades away. But it's not nearly as apparent as with the keyboard. It is the same sort of fuzziness I hear with the keyboard however. The hum would be acceptable to most guitarists I believe, especially a typical garage band guitarist. Maybe not in recording studio however.

Would be interesting to plug in a CD player but I'll need some adapters for that.

Looks like my keyboard is just not compatible with this amp, or any amp for that matter. Works great with my home stereo however.

[Chuck H]

The amp has more continuous hum when the guitar is plugged in,...

Might be normal because of noise picked up by the guitar. But it might also be ripple from the power supply evident when the shorting jack to the preamp is open.

...but I've heard much worse. I do notice that after a string is plucked and the sound is fading down to almost nothing there is a faint fuzzy sound that becomes more obvious as the string vibration fades away. But it's not nearly as apparent as with the keyboard. It is the same sort of fuzziness I hear with the keyboard however.

This is cause for continued suspicion of the amplifier I think. It should just amplify the note cleanly all the way out. I knew a guy with that amp (a very long time ago, but). I remember it being a nail for both reliable performance and reliability.

I'm thinking you may try replacing the electrolytic capacitors in the amp. Anomalies in the power supply as well as, perhaps, high impedance to HF there could be causing the fuzzy artifact as well as the hum you're hearing when the preamp is engaged.

[muelfo]

I purchased the highly recommended Peak ESR70 capacitor tester. I checked the power supply capacitors in-circuit. C61 and C69 checked good (~2500uF, 0.02 esr). For C60 and C62 the ESR70 said, "In-Circuit/Leaky".

I replaced the four power supply capacitors with new Nichicon equivalents. This made the amp slightly less noisy when idling (no music played). The faint fuzziness/distortion when notes are played is still there.

Ironically, the ESR of the original capacitors measures lower than the new Nichicons (all measured out of circuit) . The original capacitors also measure slightly higher capacitance values, but within 20% of stated value.

Two of the new capacitors are warm to touch in operation. I didn't notice this with the originals. Is this normal?

Voltages measured across the new capacitors are still good (about 41 and 15 volts).

I checked all the other capacitors in-circuit. All but one were ok according to the ESR70. The one in question is capacitor C30. It reads, "Open Circuit/Low Capacitance". C30 nominal value (0.47 uF) is outside the range of the ESR70, so that explains the questionable response.

In summary, little if anything was gained by replacing the power supply capacitors.

All comments are welcome and appreciated.


Here are all the capacitor readings measured with the ESR70:

cap# /Nominal uf/ Measured uF/ ESR (In-Circuit)

c61, 2200, 2548, 0.03
c69, 2200, 2546, 0.02
c62, 22, "leaky", 0.73
c60, 22, "leaky", 0.72
c39, 2.2, 2.31, 3.2
c56, 47, 49.86, 0.32
c24, 2.2, 1.99, 2.0
c31, 10, 10.73, 0.82
c19, 2.2, 2.52, 2.4
c30, 0.47, "open circuit/low capacitance", ???
c18, 2.2, 2.45, 2.7
c12, 2.2, 2.31, 1.84
c40, 2.2, 2.62, 2.4
3c10, 2.2, 2.39, 1.73
3c5, 2.2, 2.35, 2.8
3c4, 2.2, 2.49, 1.62
c55, 22, 21.76, 1.04
2c10, 2.2, 2.2, 2.8
2c5, 2.2, 2.26, 1.76
2c4, 2.2, 2.4, 2.62
c43, 22, 21.61, 1.04
1c10, 2.2, 2.06, 2.9
1c5, 2.2, 2.45, 1.74
1c4, 2.2, 2.28, 3.0
c45, 2.2, 2.32, 1.59
c44, 22, 22.06, 1.06

New Capacitors (Out of Circuit)
c60, 22, 21.6, 1.42
c62, 22, 21.6, 1.38
c61, 2200, 2263, 0.18
c69, 2200, 2224, 0.18

Old Capacitors (Out of Circuit)
c60, 22, 21.18, 0.84
c62, 22, 21.42, 0.84
c61, 2200, 2433, 0.02
c69, 2200, 2466, 0.04

[Helmholtz]

Two of the new capacitors are warm to touch in operation. I didn't notice this with the originals. Is this normal?

That's not normal. Check for correct ecap polarity. Wrong polarity can damage the cap.

Your out-of-circuit cap/ESR measurements all look good (old and new caps). In-circuit measurements are always less reliable, meaning that if a cap tests good in-circuit it can be assumed to be good, if not it needs to re-tested out-of-circuit.

It seems that the tester doesn't test capacitor leakage which also could cause application problems.

[Enzo]

In low voltage amps like this, I rarely encounter leakage as a problem. I mean in the sense of tube amp cap leakage.

And in fact bad caps would not be high up my list when the problem is "fuzzy".

[Helmholtz]

In low voltage amps like this, I rarely encounter leakage as a problem.

But we have two caps self-heating.

[Enzo]

And as you suggested, they may be backwards. And that should be corrected.

But the amp was having the symptom before he changed all the caps, and they were not heating then. A correctly built amp that failed, in my experience, will not get fuzzy in the manner described by way of leaky caps. Someone else may have different experiences.

[Helmholtz]

My comments were solely cap measurement/meter (not fuzz issue) related. I wouldn't suggest to replace caps without clear indication.

[Enzo]

I don't think we are disagreeing on anything in particular other than the topic.

My comments were only aimed at the repair in hand.

[Helmholtz]

I don't think we disagree on the topic either and am sure it is in the best hands with you.

[g1]

For C60 and C62 the ESR70 said, "In-Circuit/Leaky".

This means the capacitance could not be measured in circuit so it needs to be rechecked out of circuit. It may be leaky, or there was a parallel circuit path that would not allow a capacitance measurement.
The ESR number will always be valid in or out of circuit.

[muelfo]

And as you suggested, they may be backwards. And that should be corrected.

But the amp was having the symptom before he changed all the caps, and they were not heating then. A correctly built amp that failed, in my experience, will not get fuzzy in the manner described by way of leaky caps. Someone else may have different experiences.

I'm not certain he original caps weren't getting warm. I didn't think to check, but the new C62 it too hot to keep my finger on it. It's situated pretty close to a couple of high wattage resistors, so maybe some heat is coming from there.

I very carefully oriented the new caps in the same direction as the ones I removed. I've also confirmed the orientation agrees with the schematic.

I'm beginning to think the main problem is that this Peavey "keyboard amp" is not designed to work with a keyboard like mine (Yamaha P85). The amp seems to work ok with a guitar. Maybe the amp is designed for keyboards that have no preamp. In other words, keyboards that produce "line level" signals at the output, not headphone level.

Also, I may be unfairly comparing the sound performance of this Peavey amp to the more electronically sophisticated amp I normally use. I've always played my keyboard through my home stereo system which has a fairly high quality JVC AX-R97 integrated amp with 125 watts per channel. My speakers are JBL Century L100 studio monitors, a rather legendary design in the audio world, or so I'm told. There's no hum, hiss, or distortion at all that I can hear. The Peavey circuitry looks like a tinker toy in comparison. The Peavey sound is not in the same league as the JVC. But maybe the Peavey is designed for use in rock bands where distortion is an advantage. I'm sure the Peavey can withstand physical abuse however. And that's why I'm interested in it. I wouldn't take my home stereo "on the road" so to speak.

I recently found out that I know someone who has a Peavey keyboard amp, just not same model. We're going to try my keyboard with that and see if it does any better. Thank goodness I didn't buy a new "keyboard" amp before finding out my keyboard may be incompatible.