I am knee deep in a project to put the Lab Series L5 preamp in a 1790NS enclosure (EHX "XO" series, or around the size of a Fulldrive)

It'll be pretty well identical to the original, except it'll be channel-switchable, with the initial gain stage shared between both channels and a footswitch to switch it from there. I am also replacing the CA3094 with a CA3080 + discrete Darlington per the original schematic in the 3904 datasheet.

Anyway: I have a couple of questions regarding power handling and biasing.

First: the supply. I don't think there is any good reason to stick with the +/-15V from the original, when compared with the difficulty of making such a supply (not to mention the danger of "rolling your own" from AC power). I am planning on using a LT1054 voltage inverter that should work with either 9V or 12V.

The LT1054 can handle up to 100mA. So, I crunched the numbers on the L5 and came up with the following power requirements:

CA3080: 1.2mA each (x2) = 2.5mA
LM741: 3mA each max (datasheet says 1.7-2.8mA) (x2) = 6mA
JRC4558D: 6mA each (3.5 - 5.7 datasheet) x12 = 72mA
LF356N: 10mA each (5-10 datasheet) x1 = 10mA (max)

Total: 90.5mA if everything was at the very top of its specified range. This is also based on their power requirements assuming +/-15v. The draw should be around 20% less at +/-12V and 40% less at +/-9V, so the full circuit should be significantly less than the LT1054's capacity.

So my first question is: Should this theoretically work OK? Or have I made a mistake in my calculations someplace?

Second question is much easier. The trimming/biasing/tuning methods for the L5 are well documented in their service manuals, and the schematic itself is loaded with test voltages at major junctions and test points. If I am using a supply lower than the standard +/-15V, should I aim for the voltages and test results they specify (since we are nowhere near the rails no matter if it's running at +/-9v, 12v or 15v) - or should I plan on everything being proportionally smaller?

http://www.netads.com/~meo/Guitar/Amps/Lab/Images/Lab_Series_L5_L7_L9_L11_scheme.jpeg

I'm in the process of designing & building a Lab Series L5 preamp clone to put in a rackmount enclosure. I think I have everything else figured out but this: I want to remove the reverb from the circuit entirely. I think I know how to do it, but I'm looking for some independent verification that I'm not missing anything!

If I was to eliminate the wire labeled "4" that goes from the preamp board to the power supply board, and eliminate R165 (18k) that goes between the multifilter and the reverb pot, and then remove the whole section between wires 5 & 6 and R165, would that do it? Do any of those components serve a purpose other than reverb, such that I would lose out on anything if they weren't there? Is the multifilter control affected at all by this?

Everything else related to the reverb appeared to be on the power supply board itself, which I won't be using...

I'm building a preamp that needs a 15v bipolar supply. The catch is that I have never built anything that required a transformer; all of my electronic adventures have been solidly 9 volts with a wall-wart. So I'm looking for some advice!

The preamp will be in a 1U rack case, so the height of the transformer is important.

I will probably be using this as the project for the power supply:
http://www.paia.com/proddetail.asp?prod=K83&cat=50

I did some poking around and found this transformer, which looks like it'll do what I need, for a very reasonable price:

http://www.mouser.com/ProductDetail/Hammond-Manufacturing/164G20/?qs=sGAEpiMZZMtz8P%2feuiupSThvwaRcnmu81WhoAji7k8o%3d

I also found the ST-5-20 on this page, which is very similar in specs:

http://www.signaltransformer.com/content/white-goods-splittran-low-power-transformer

Is one of these what I need, or is there something better I should use? I don't think the preamp will pull more than about 200 mA based on the components, but I went with a 600mA supply just to be safe. They are both PCB-mount, but I figured I could just use a piece of perfboard for that.

From what I understand, a Darlington transistor is basically 2 transistors in one, where the second one amplifies the first. So even though this particular part, the D16P1, is long gone (can't even find them on ebay), there should be a way to recreate its functionality with two readily available transistors based on the specs, right?

Here's the best datasheet I could find:
http://www.datasheetarchive.com/pdf/getfile.php?dir=Databooks-3&file=Book517-24805.pdf&scan=

How would I go about finding which two transistors I could use for this? Or is there a single-package Darlington with similar specs still being made that could substitute? I looked at the MPSA12 and similar ones, which are pretty commonly used, but most of the specs weren't anything alike, and I don't know which specs are important and which aren't.

I've made 3 or 4 posts here about my Lab Series L5 amp since last year... I think I finally fixed it yesterday. This year-long process has taken me from clueless beginner to somewhat competent near-intermediate, but I'd still like to learn all I can from this amp before I put it back together.

I have the service notes for the amp which give details on how to set the bias, the offset, the distortion trim and the compressor level. I'd like to learn to do all these things but I don't know where to start. I think the concepts will be pretty easy to explain, but I am lacking a basic educational foundation for most of this and so I'm finding it hard to know exactly what I need to do when faced with these instructions. I've been scouring Google for a couple of days trying to find info on this so I could teach myself, but all search results are flooded with tube amp adjustment information that's not really the same thing.

So anyway, here are the four adjustments from the service notes, with each one followed by my questions for clarification.

BIAS ADJUSTMENT
 L2, 4, 5, 7, 9 & 11 bias adjustment is necessary if Q309, 311, 312, 313, 314,
 output transistors or driver transistors are replaced i.e.:
       Q307 & pre driver
       CR310, 311 bias diodes
       Q308 pre driver
       CR 305 3-layer diode
   1.  Remove fuse F21 inside power supply and insert a D.C. Ammeter, set on
       the highest scale and set the input to meter the edge of power supply.
   2.  Turn on the unit with no signal input applied.
   3.  Allow the amplifier to stabilize for a minimum of 3 minutes, but ideally
       at 10 minutes.
   4.  Set R311 for a reading of 200 ma.
       Note: Unit must be at room temperature before beginning the bias adjustment.

I think I understand this one: I should remove the fuse and clip one probe from my DMM to each of the fuse mounts in its place. Is that correct? Does it matter which probe I use for which side?


OFFSET ADJUSTMENT
 L2, 4, 5, 7, 9 & 11 (best done after bias adjustment is completed)
   1.  Insert an 8 ohm dummy load capable of handling full power of amplifier and
       with the master volume CCW, adjust R306 for 0 volts +/- 10 mv. (across load
       yellow wire on power amp)

I don't have a dummy load but was thinking of building one. But what purpose would a dummy load serve if this adjustment should be done with the volume at 0?

Second, where do I put the two probes to measure the voltage? It says "across load yellow wire on power amp" which is not the clearest description (this document was pretty hastily typed in the 70's and I've noticed a number of other mistakes as well). Does this mean to measure the voltage from the power amp's yellow wire to the load itself?

DISTORTION TRIM ADJUSTMENT
 L2, 4, 5, 7, 9 & 11 (which simulates pseudo-tube distortion which flat tops the
                      waveform)
   1. Insert 30 mv RMS (-29.6 dB) 1 KHz sine wave on pin 2 of A110, 3094 I.C. and
      adjust R171 for 4.4 V (+15 dB) on pin 6 of A110.

First I downloaded a sine-wave generator program, then I configured it to generate a 1KHz sine wave. I turned up the volume until I measured 30 mV across the tip and the sleeve of the cable coming from the sound card. Is this the correct place to measure it? The sound level in the program was very near -29.6 dB so I think I was doing this right, but I wanted to make sure.

Next, it says to "insert" the sine wave on pin 2 of A110 and measure the voltage on pin 6. What I did was clip the red probe to pin 6 and the black probe to the chassis, then I turned on the amp, played the sine wave on the computer and touched the tip of the cable coming from the computer to pin 2 of the op amp. But I only measured a few mV on the DMM, nowhere near 4.4V, and I was getting a lot of ground signal out of the speakers with only a very slight amount of the sine wave signal. What was I doing wrong? What's the correct way to "insert" the signal to pin 2?

COMPRESSOR ADJUSTMENT
 L2, 4, 5, 7, 9 & 11
   1.  Connect a dummy load capable of dissipating full amplifier power (8 ohm)
   2.  Tone controls set flat, the master CW, and the compressor turned off
       increase the channel volume until full power is reached:
           100W @ 8 ohm  28.3V RMS, 80V pp., +29 dB or
           200W @ 8 ohm  40V RMS, 113V pp., + 32 dB as applicable
   3.  Insert a 1KHz sine wave  100 mv  -20dB into the input jack of either channel
       ("Lo" jack).
   4.  Turn compressor switch on and set at "0".
   5.  Adjust R186, the compressor trim, until the compression LED just turns on.

When it says "tone controls set flat", does it mean 0, 5 or 10? Except for the midrange control (which goes from -5 to +5) all the other tone controls are passive. I assume this means 10 (on all but the midrange) but I wanted to be sure.

Anyway, thanks in advance for any help you can offer... This will go a long way in helping me to understand some of these concepts that are eluding me right now. I really appreciate it!

OK. I've posted on here a couple of times before about a Lab Series L5 that's been giving me problems. I worked on it off and on for about six months without really making any progress, and finally about a year ago I gave up and put down my soldering iron, and just used the amp as an external cabinet.

But it's not in my nature to just let broken things sit around, and a week ago I was doing some unrelated soldering when I decided to have another go at it.

The good news: I fixed one of the problems, an inexplicable constant crackling sound that was unaffected by the master volume control. I replaced CR101 and CR102 (1N4004 diodes) and it went away. Very cool.

But there was still another problem, a bad compressor that made a loud popping sound every time it kicked on (i.e. when the signal went over the compressor's set threshold). That was detailed in this thread from last year. (I said at the end that it was fixed, but the problem came back about a day after I posted that, which was right about the time I gave up on it.)

All symptoms pointed to the power supply, since I replaced just about every part in the compressor's circuit on the preamp board and nothing changed, plus the LED flickers off when the compressor comes on, like there's insufficient power to supply both the LED and the compressor itself. So I replaced all the 1N4004 diodes on the power supply (couldn't hurt, and I had a bunch of spares), and I strengthened a torn trace coming out of one of the TIP30 transistors. (I had fixed it before, but the two components were only connected by solder, so I put a copper wire over the trace between the parts.)

That's all I did. But I somehow created more problems. The amp makes a loud crack when it first turns on, then it makes a steady, rhythmic popping sound all the time that it's on. And when I turn it off, it makes a very loud pop. I've only turned it on and off three times since messing with the power supply board, but it seems to me that this pop has gotten louder each time. I'm now pretty concerned about turning it back on again at all.

Anyway, I made a recording of it this last time I turned it on, which I've attached to this post as a zip file. This was recorded using an iPod, and it was a bit too close to the speaker, so the crack when it turns on and the crack when it turns off are heavily clipped, but the steady pop is clear.

Isolation details: The cracks when the amp turns on and off are unaffected by the master volume. Volume was at 0 when the amp turned on, then I raised it to listen to the rhythmic popping, then I turned it back to 0 before shutting it off. The rhythmic popping sound is affected by the master volume, but not by the channel volume of either side. Both of these were at 0, and I just raised and lowered the master volume for the sound recording. The loudest I turned it was about 3.

WHAT IN THE WORLD IS WRONG WITH THIS AMP? I am pretty good with a soldering iron so I don't think it's due to a bad soldering job anywhere. I very meticulously researched replacement parts and only used the exact part number, never even an equivalent, and I was very careful that every replacement went in the same way its predecessor came out. I took plenty of pictures before I removed any parts to ensure this. This is just baffling, as well as frustrating, because I feel like every time I do something, something else goes wrong.

For reference: board layout and schematic can be found here -
http://www.netads.com/~meo/Guitar/Amps/Lab/schem.html

OK... as a preface, this isn't actually a guitar amp, it's an active preamp inside of a guitar. But the principles are all the same! I need help trying to figure out why two preamps with the same specs sound differently. I have attached schematics for both of them.

One is from a Fender Elite Stratocaster from the early 80's, and the other is the Eric Clapton mid-boost from the late 80's continuing to today. The Elite came first and then the Clapton boost was based on it. The two guitars sound very different, and though this could be caused by the body wood or the pickups, I have a sneaking suspicion that the preamps are actually voiced a bit differently.

I am not well-versed in schematics, so this is like requesting a translation, but I was mainly wondering if someone could compare these two schematics and tell me if there's anything obvious that might cause the two boards to sound different. It seems like different frequencies are being boosted, and it may just be a matter of switching out a couple of components for different values to get the change in sound. (The full story is that I have the newer Clapton preamp and I want it to sound more like the Elite, and was wondering if I could do it by swapping a couple of twelve-cent components.)

A couple of notes on the obvious differences I found:

- The Elite diagram has a whole section for the dummy coil which cancels out hum in all positions, as well as a buffer to drive it. The Clapton model does not use a dummy coil as it's fitted with noiseless pickups, so this difference can probably be ignored.

- The Elite uses a 50k linear taper pot for the mid-boost tone control, whereas the Clapton uses a 250k audio. What effect would this have on the sound? May account for some or all of the difference.

- The Clapton ends up with a 25dB boost while the Elite only has 12dB. From what I've read this is just a difference of a couple of resistors. The total output is not an issue, the idea is just to compare the sound at the same boosted dB level.

I have a number of other documents I can upload if it would help - a parts layout for both boards and a detailed closeup of the Elite circuit board, front and back.

Thanks for any help you can provide! As I said, I'm fairly inexperienced in figuring this stuff out for myself, but I am very curious to see whether the boards are voiced differently.

OK... so my Lab L5 has been giving me some trouble lately... it would cut out intermittently, so it was too unreliable to play live. Long story short, I narrowed it down to the power supply board causing the problems. My weekend project was to replace the six transistors (Q201-206) to see if that fixed it.

While I had it open, I decided to put sockets on all the opamps on the preamp board so I could experiment with different chips and swap them out easily. When I tried to put it back together, it just barely didn't fit in place because the sockets made the chips too high... so I had to take them all back out. Save yourself the trouble and don't ever try putting sockets on the preamp board

Now, the compressor pops every time it kicks on, and it quiets the sound and then gradually brings it back up, very similar to a Slow Gear. I'd be tempted to think something was wrong with the compressor opamp, but I switched it out with a replacement a couple of times and resoldered it a few more times and it always did the same thing.

My suspicion is that it's actually something in the power supply causing it, because the weird behavior is directly related to the compressor's LED turning on and off. I know LEDs draw a lot of current when they first turn on, and I'm wondering if something could be wrong in the power supply which would cause it to not be able to supply the necessary current.

The service manual has this to offer:

F.  Distortion or Popping Sound
1.  Out of adjustment.
2.  Compressor component failure problem of A112, 4558, A111, 3080 or CR101, 102, 1N4004 leaky & introducing ADC voltage on the input.

I have spares for each of the components it says to check, but the compressor has never given me a bit of trouble before I replaced the components in the power supply, so I'd like to get some opinions before I go switching out those components. Any ideas on other things I can check?

I have a 1980 Lab Series L5 that's been giving me a lot of trouble lately. After resoldering the preamp and power supply PCBs and replacing a couple of components that looked weak, I've brought it down to the following two problems:

1) A constant "crackle" sound. It's not a hum, hiss or popping sound, but I don't know how else to describe it. It fluctuates in volume – sometimes it's pretty loud and sometimes it's barely audible, but it's always there. The noise level stays the same regardless of the master volume, so I think this is coming from someplace in the power supply rather than the preamp. It still processes input signal just fine, but that crackling is very unnerving! Does this sound like a problem with a particular component? The next step is replacing components one by one which I'd rather not do if I can avoid it...

2) The compressor sounds like it's on its last legs. Whenever it kicks in and the compressor LED shows action, it cuts in and out very erratically until the signal decays below the compressor's threshold. This is a new problem. It sounds like it might be a bad solder joint or something, but it started happening about three weeks after I resoldered all of the joints on the preamp. It's fine if I turn the compressor switch off. Would this be fixed by replacing an opamp someplace, or is it more complicated than that?

I found a few older threads on this board about Lab Series troubleshooting, and it seems like there are a few people hanging around who know a lot about these amps. But just in case, here's a link to the schematic, board layout and service docs:

http://www.netads.com/~meo/Guitar/Amps/Lab/schem.html

Thanks everyone!