Lab Series L5 preamp in a pedal

[stringsthings]

Really nice project.  It's very hard to resist !  ( lack of current funds )

[gordo]

L5 is up and running.  I need to chase down an intermittent solder joint and I totally missed the fact that the voltage regs mount face down instead of face up (so the tabs fit over the diodes...duh...) but other than that it was a long drawn out build that fired up nicely.  I haven't messed around setting the trimmers yet, I was too busy screwing around with it.  It feels like the real thing and I can't wait to get my real L5 home to compare them.

I'll order an enclosure this week so I can get it boxed up.  I haven't been this excited about a pedal in a very long time.  It is a very serious commitment in terms of build time and money but no more so than building 3 or 4 of the boards we normally deal with. If you've successfully done a Laserwolf or any of the Bean big boards this is on par.  Just BIG.

Just an insane amount of work had to go into laying this thing out and major kudos to Aion for all the effort.

[DragonSF]

I finished to build this beauty (even my wife likes it - just from the looks ), but I have a small problem: the IC 5 (Mid2) is clipping the input signal. Can someone please compare the voltages to a working one?
I got:
1: 0.29
2: 0.01
3: -0.195
4: -15
5: 0.175
6: 0
7: 0.9
8: 15
The clipping starts, when the pot Mid2 is half open.
CH 1 is working fine and I had too the problem with the direction of LM 7815 and 7915.
     

[aion]

The voltages look OK, theoretically anyway (I don't have mine in front of me to check at the moment). But, without hearing it, I would say that there is a chance this is normal - channel 2 distorts really early and there will pretty much always be a crunch to it unless the channel volume is way, way down and the master is way, way up. In my experience the clipping circuit is particularly sensitive to the midrange, and the EQ circuit gives you a ton of control over the midrange, so depending on where the mid frequency knob is set, this can sound really good or really bad. Channel 1 is much easier to get a clean sound from.

At the least, though, I would double-check the distortion trimmer against the setup procedure and make sure everything checks out with that, since that affects how soon the OTA goes into clipping. If it sounds like something is wrong (un-musical) then it may be that the trimmer needs to be raised.

[DragonSF]

Actually, my observation was wrong. The part does not distort the original signal, but creates an own 4Khz signal 'triangle wave), which is distorted/clipped to a rectangle signal. The frequency varies a little with the setting of the Mid2 pot.
Musically not really useful.

[DragonSF]

I eventually found the problem: Because I couldn't get a proper dual gang  pot C100K for PCB mount, I had to build one from a lug pot. During the tedious inserting of the PCB in to the case, one of the middle joint was broken. After resolder the joint, everything is fine.
Some advises: using MTA .1 connectors for all PCB connections helped debugging. No need for unsolder, resolder etc. Just unplug and replug. Luckily Kevin had the inspiration to use the .1 spacing for the terminals.
As I couldn't get the pot protectors, I used plastic coated tape to protect the PCB from the pots. This was not good enough, so I added some heavy sheets of plastic between pots and PCB. This was effective.

[markeprice]

Sounds a bit off but, I cut little circles from engine gasket material to use as insulators on the back of pots.  Pretty cheap, I just glue to the back of the pots.

[DragonSF]

Anther strange behavior (which happens only with the internal power supply): when the limiter kicks in, the V- shows ripples of 25Hz as if overloaded. This is audible at the output.I.e. when the limiter kicks in, the comp LED gets on on only the mains hum (half the frequency) can be heard. The voltage over R95 gets to 12V at this point. This would be around 20mA. According to my measurements, the normal load is around 75mA on the V- rail. With limiter, it's 95mA.
That's way below the limit of LM7915 (1A) and my AC adapter (1A). As above stated: external +15V, -15VB: no problem at all. Positive current is constant at 75mA. Any ideas?

[aion]

What electrolytic caps did you use for the PSU?

[DragonSF]

The same ones, which were on your Mouser BOM. I replaced C60 with a 220u and then with original value, different voltage to no avail.

[DragonSF]

It's unbelievable, but eventually I found a solution: I replaced the 9VAC PS with a 12VAC and the noise is gone.
Explanation: I think the output voltage was on the border line. We have 100V here in Japan, but the PS was for 120V. That means, that the 9V was really 8V, which is sufficient for a certain current. If the current is higher, then the input voltage is not enough for the 7915 to be regulated to -15V, so it was dropped. Maybe this is far fetched, but sounds plausible to me. Please correct me, if I'm wrong.

[alanp]

Just had a look at the schematic... 

15VAC would be better. Going from 9VAC to +/- 15VDC is probably pushing your luck. I'm guessing that C51 and C52 are playing a massive part in why the PSU actually works to begin with. If you were feeding 9VAC through a diode without the caps, getting... less than 9VDC into the regulator... normally, a 7815 would go weird, and certainly not put out 15VDC.

I'd love to hear comments from one of our forum guru's on the power supply section of this pedal. I'm genuinely interested, now. Most of the linear PSU's I've seen, you need a few volts higher going in than what you want post-regulator, not less than the regulated voltage!

[aion]

It's actually a voltage multiplier supply - that's what the extra diodes and caps are doing there in the first section before the regulators.

9vAC source supply, multiplied by 2 from the doubler, multiplied by 1.4 to convert to DC, so the regulators see about 25V DC. Then it's pulled down to 15v DC from there.

More info here:
http://www.play-hookey.com/ac_theory/power_supply/ps_v_multipliers.html

[gordo]

Just got my enclosure in from PPP yesterday.  Looks fantastic and the color is just obscene.  I'll get some pics up when I get it drilled and the guts are loaded.  I'm also currently without phone and internet at the house so feeling a bit like Fred Flintstone these days.

And most importantly...got Jolly Ranchers with the order.

[DragonSF]

I found the best place for the pre-amp in the effect loop of my KPA.