Building overdrive from this?..more treble?... I've traced a Levin amplifier

[teemuk]

That "response similar to what many tube amps have.." sounds interesting

See especially section titled "Mixed mode feedback".

http://sound.westhost.com/project56.htm

If you download a bunch of guitar amp schematics a good chance is that you'll bumb to this type of circuit. It's been a common trick since about mid 1960's. Now it's almost a standard in guitar amps.

Looks like the image of the component list was a lot more accurate considering component values. It seems you made a few errors when updating those to the "trace image". After seeing the Crate schematic I doubted if some of the values in the sketched schematic were correct (e.g. that "treble" cap in the tone stack and the 10F cap in the second distorting stage). Seeing the component list assured the ones in the Crate schematic were indeed closer to truth. Anyone who wants an accurate schem should use the component list as a reference.

http://www.4shared.com/file/240656908/44810660/Crate_GX-15.html

[ericohman]

I'll look into the parts and post and updated, hopefully complete and corrected "trace image".

I edit my last post on Page 1 and added a photo which you might have missed, because I think we posted almost at the same time.
It was just a close up of the pots with Hz values for the EQ section.

About the EQ section:
The sound clip I posted had these settings:
(Pots 0-10)
Bass: 1 (there's extreme amounts of bass in the overdrive channel, if you have the pot turned halfway or higher it's just crazy)
Mid: 5 (the mid control is alright)
Treble: 10 (I would like to see this pot go to at least 15, when you rotate the knob you can't really feel any treble shining through.. almost useless)


Regarding the "1µF" after the gain pot, it is supposed to be .1µF
Sorry for not darkening the pcb enough to make the dots more visible. I'll fix everything to make it more clear and also with the missing diodes.

[teemuk]

I happen to have GX-15R as a practice amp and I've noticed the very same thing. The OD channel is just buzzy mess.

Some issues explain it:

- Channel sharing and simple design: There simply isn't much of pre-gain voicing going on in this amp. That would have involved additional gain stages or alternatively tampered the voicing of the clean channel. What you have is mostly a compromise. For example, Ampeg VH-140C has a whole gain stage that boosts the mid frequencies almost a ridiculous amount preceding the distortion stages. This little amp on the other hand has only a gain stage with rather flat response. Thus, the signal hitting the first distortion stage does not have a lot of attenuation on low frequencies. The clipping of low frequencies tends to "mask" the high frequency information and the tone easily turns into bass-dominant mush. There's some low-freq cutting going on but it clearly isn't enough in this circuit. You could try increasing R6 (which cause low frequencies to have less gain in the first distorting stage) and decreasing C7 (which is a high-pass filter for the second distorting stage). If that doesn't help try reducing the amount of treble cut: decrease values of C6 and C9.

- The tone controls: Once again they are shared by both channels so there can't be too drastic shaping going on since it would mess up the clean channel's voicing. You're again dealing with a compromise. Compare this to e.g. Ampeg VH-140C that has separate clean and OD channels and separate tone controls for both. Not to mention the controls are voiced very differently. The tone controls in this amp are also post-distortion so they can't alter the signal hitting the distortion stage. Post-distortion controls are more effective for overall tone shaping but If something's gone wrong before them (e.g. distortion is pushed into bassy mush) it's too late to try to save it now. Reducing bass or boosting highs (which is pretty much the same thing as the former) aren't very effective techniques because most of the high frequency content has likely been "devoured" due to clipping of the low frequencies. There's simply nothing left to boost back anymore.

This is why modern hi-gain circuits actually hit the distortion stages with a signal content squeezed into a very narrow bandwidth, or at least hi-pass filtered drastically. This is why guys always used treble booster pedals to drive old tube amps that weren't yet voiced for high-gain.

As for the tone controls in this amp. They have a range of about 20 decibels and are rather effective. It's a basic circuit so you can input the values to a software like Duncan's TSC and see how the controls work frequency wise and waht component value tweaks will cause.

I actually like the clean channel of that little Crate amp but the OD channel is just pathetic.

[ericohman]

I agree with you, except that I like the OD channel, and even if the EQ section is far from perfect in OD, I hear potential, that's why I started the thread.
I'm interested in tweaking the EQ so that it works better for the OD channel. Maybe I'll try to build a clone of this amp but only the overdrive, no clean/od switch at all, with a modded post eq-section to start with. Or, as mentioned, try to build some kind of stompbox, based on the circuit.

I just finnished these. This time I'll just link to the pictures because they are so big (1280 wide)
I had another look at the diodes and I could see the number on 2 more diodes, but still 2 diodes that I can't read the whole number on.

NOTE that there were two C3's before, now one of them is C8 which is correct, it should be a 3,3µF.
Also, there were two D4, I changed one to D4b and I also noticed that due to the duplicated D4, I had made a mistake on the computer putting the diode number for the other D4.

So, this is close to something I'd call Final:
http://djoman.dyndns.org/levin/images/levin_retro_001_v002.jpg
http://djoman.dyndns.org/levin/images/levin_retro_002_v002.jpg
http://djoman.dyndns.org/levin/images/levin_retro_003_v002.jpg

Will have a look at it tomorrow after a night's sleep.

[ericohman]

What diode could part D2 and D5 be?
http://djoman.dyndns.org/levin/images/levin_retro_002_v002.jpg
http://djoman.dyndns.org/levin/images/levin_retro_003_v002.jpg


You can only see T41, and on other diodes I could read 4148. Could D2 and D5 be 4148 as well, I mean, T4148?
Would be nice to make a final version of the schematic.

I'll try to do some "photoshopping" with the photos to make a pcb with holes (0.7mm) and board measurements if anyone ever wants to try and build this Levin Retro 15.
Just need to take another photo of the traces with a tele lens to get rid of the parallax effects that occurs when shooting closeups.

[teemuk]

What diode could part D2 and D5 be?

You can only see T41, and on other diodes I could read 4148. Could D2 and D5 be 4148 as well, I mean, T4148?

With most certainty they are both 4148. In both cases the design just called for a generic silicon diode, which for Philipson meant their big stock of 4148's. It doesn't really matter what you fit there as long as the forward voltage is about the same (0.7V). Under normal operating conditions the D1 and D2 pair isn't even supposed to conduct, as they are an overvoltage protection for the input.

[ericohman]

Okay, thank you!

I took your schematic and updated the parts that were mistakes by me, as well as updated it with a parts list.

A few questions popped up...
What wattage is this resistor, and what TYPE of resistor, metal?
Part number R23, to the right above the phones jack:

http://djoman.dyndns.org/levin/images/levin_retro_003_v002.jpg

I tried to google subs for the IC1 but not luck at all. Does anyone know a good site with a substitute database?
Would be nice to have subs for the KA3403 quad op-amp.
Here's the datasheet:
http://www.datasheetcatalog.org/datasheet/fairchild/KA3403.pdf

The TDA2030A seems more common, but subs for that part may also be of interest to someone...

Last question, one pot only said B500 and I noticed that you (teemuk) wrote it as B500RB, what does the B in the end stand for?
Here's the pretty much complete schematic, with parts reference below, as well a part list for ordering.
Sub part for the KA3403 wouldn't hurt to include in this picture.. as well as the watt for the 220 R23 resistor.

http://djoman.dyndns.org/levin/images/levin_retro15_schematic.gif

[ericohman]

What is the purpose of D1 and D2?
I've noticed it says V+ and V- on a few places other than at D1 and D2.
For example the IC1 has V+ and V-

Does that mean that a need bipolar supply, like a +18V and -18V?

http://djoman.dyndns.org/levin/images/levin_retro15_schematic.gif

[klangfix]

Hi,
This is my first post here.

I thank you very much for the tracing of the Levin 15R Retro. I am trying to modify a Levin 50R, which is slightly more complicated but not very much. It has two more OP amps: JRC4558DD and the power amp consists of two transistors TIP41C.

My plan is to change the elctrolytic capacitors to double the value: 10000uF and 50V with low ESR to get rid of 50Hz hum.
The second modification is that I want to reduce noise by changing to modern OP amps. I was planning to use LM837N and NE5532, but for some reason the NE5532 does not work in the same way as th JRC4558, and I cannot really understand why. I thought they were pin compatible. Now, the sound is extremely distorted.

Any idea?

And BTW, I have exchanged the short 2-spring reverb tank to a much better long 3-spring reverb tank from Acoutronics. That was a great enhancement.

[Quackzed]

another way to bump up the treble in the posted schem from

Schematics and other junk levinamp

would be to reduce c2 and maybee also c5 and c8, c2 especially is larger than usually seen in the feedback loop of the first op amp. this usage of caps in the fb loop is generally to cut 'fizz' or noise but c2 at 1n is  larger than the normal 50p-250p and cuts alot of high end, also c5 and c8 could be reduced to 50p or 150p if its still too dark...
this might be a win lose however as these caps may increase treble and as a consequence increase harshness, so you might need to find a happy medium between 'not trebly enough' and 'too harsh/edgy' but lowering these WILL let more high end through for good or bad ...