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2.5ch preamp build


Zozobra

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As I promised casey in his thread here is my preamp build.

 

Brief:

Its a 2.5ch affair where there is a common boost stage for each channel. The clean is pretty much taken from a VH100R as that has a great clean IMO, dirt channel is fairly generic three stage cold clipping and I'll no doubt play around with the values in there quite a lot when I get the thing working. Boost stage has a mid/full boost switch to select the bypass capacitance for lead/rhythm work. The channel and boost are all footswitchable using relays.

 

A shout out goes to PaoloJM for helping me with the switching and to olaf (VH4) for donating an output stage. Much appreciated guys. I may be cutting it a little fine with the heater current to run three relays so I may have to get inventive...

 

And now, the schematic:

 

preamp_rev_4.png

 

If thats too small a hi-res version can be found here.

 

A few other piccies:

 

46157_10150267016165601_727505600_146234

PT (from ampmaker.com) and the tag board I'm building on

 

61080_10150287570850601_727505600_151192

Chassis is it grey primer glory. This will be sprayed white after its been drilled.

 

More stuff to follow. Going to put the order for the last few part in tomorrow and they'll hopefully come within a week or so which should be enough time to motivate me to redo the layout and drill plan!

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Yeah, well apart from a little plagiarism and advice ;)

I bought merlin blencowes book earlier in the year and I learned a hell of a lot from it. I dont think I've seen anyone say a bad word about that book. I'm certainly interested to try the LED biasing out after I've got it working using the resistors/caps. I know its popular with hi-hi builders as you get full gain without the sluggishness of the bypass cap.

 

Still disputing what voltage to put in the plates. I'll get 320V or so after rectification and I want 300-310 for the clean channel but havent given the rest much consideration yet.

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The switching is something I'm still learning about. I may have to lose a relay due to current draw in which case I'll use a more standard line select method.

Take a look at what happens with the "Cn Sw rly A" in the position, as drawn.

 

The signal to the bottom section is shunted to ground. So that avoids driving it with a signal that isn't gonna be used at the time anyway. All well and good.

 

But look at what happens to the signal that continues through the top part of the circuit. The 10k resistor and 22nF coupling capacitor form a high-pass filter. Everything below the corner frequency of about 725Hz drops off @ 6dB per octave. This can't be good. :(

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Thanks for spotting that. I could have been scratching my head for a while at that one!

No worries, mate. You also have some weirdness happening at rly B. The position of VR9 is gonna affect the tone of the signal on the upper channel. More cut of the highs when VR9 is at minimum or maximum. Not sure if the effect will be very noticeable, but it will be there.

 

Is that a typo on the value of R22? Should that be 33K, or did you miss the decimal point for 3.3K?

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Cool thanks again. I'll swap the channel switching to a one relay input select later. Hopefully I wont make too much noise on channel switching. I need to add a large resistor to ground from the coupling cap on the boost stage too for this purpose. I think the channel.

 

Yup 33K, going to try a 39K too for some real cold clipping. Might end up using a more subtle value in the end though.

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Yup 33K, going to try a 39K too for some real cold clipping. Might end up using a more subtle value in the end though.

Ah, so that's what you're after. Well, the gain of the circuit should be pretty low, with all that degen happening at the cathode. Maybe like 2 or something. And the idle current should be very low, 100~150uA. Cold indeed. And close to cutoff. So yeah, it probably won't take much to drive it into clipping.

 

I've never played in this region. Dunno what it's gonna sound like. I'll be interested to hear from you after this had been built. :)

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Yeah its pretty low. Did the calculation with merlins spreadsheet and its certainly under 10 and probably even under 5 as you say. Its pretty much what mike soldano did with the SLO to give it that sound and quite a lot of high gainers since have copied that design feature. I like that sound but I dont really want a silly 5+ stage design as it robs too much of the subtlety from the tone. I like to get a good bit of dirt going on the amp and then slam the front with a TS or fuzz, sometimes a TS and a fuzz :lol:

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Good spot. Damn you're good at this! Thanks!

 

It was a pot but due to panel space being tight I decided to drop it and go with fixed attenuation but forgot to drop the ground from the bottom of it. Its meant to form a potential divider with R29.

 

I should probably add a 1M to ground from the back of C20 to stop it from poping when the boost is toggled.

 

Also I'm thinking that I may wire the C7 bright cap on a on-off-on dpdt switch so I can have two values or bright boost as the feature wont cost much extra. May do something similar with the boost voicing bypass cap too to select hi mid boost, low mid and full range.

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Good spot. Damn you're good at this! Thanks!

Some friends and I used to toss our designs back and forth for proofreads. I've gotten quite good at spotting irregularities in others work.

 

I still make a lot of errata in my own. I think I get a little too familiar and don't notice my own mistakes. I think it's common for everyone.

 

I copied your schematic and put some comments on it. Not so much errata but just things worth a little discussion. I hope that's alright.

 

ZpreampCommentsSmall.png

 

Large version

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I'll rephrase. I hate re-doing layouts. The first one I didnt wont fly with the new schematic so its back to the drawing board. I should probably save myself a lot of time and effort by making my own eyelet board rather than trying to shoehorn stuff on tag board.

 

Cheers man. We really made a mess of that recording. All done live with not much time to set up and get things done. Took an age to mix out all the bad.

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