simplest EQ and compressor?

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kiira

Well-known member
Joined
Aug 2, 2004
Messages
536
Location
Baltimore - Blobsville USA
Hi, someone asked me to build them a guitar/mic preamp with compression and EQ. I have some ideas.... like maybe a simple 12sn7 5687 tube pre with transformer inputs for mic and instrument. The simplest compressor around seems to be the PRR varimu though if anyone knows of something else I'm all ears. I have looked for some type of EQ but can't find anything. I'd like to stick something in between the input stage and the line stage. Does anyone know of something I could make on one board which would fit the bill? I could actually get paid for making this! I wasalsothinking of building one for myself.

It would go into a 1U 8x17 box. If I put tubes and transformers on the back of the box it seems do-able.

Here is a present for anyone who helps ... http://www.fairradio.com

They have some decent prices on tubes... $4.50 for NOS 12sn7 or 12sl7 for instance. Or buy this radio thingy called car-mon-87 http://www.fairradio.com/car-87.htm

For $7 you get a 300-0-300 35 mA power transformer, some AB locking shaft pots, some nice tubes and ceramic sockets with sheilds, a 600:600 AF transformer, bunch a nice resisters and caps etc. I bought 2.

I was almost done with my Telefunken V672 but then I scored this cute round VU meter so I have to take off the panel and put the meter in and make a driver board for it now.

Kiira
 
You could build a normal mic preamp with a sidechain driving an opto in paralell to the gain feedback-resistor. You could include the EQ circuit in the feedback loop also.

That would be the simplest channelstrip ever, I think... And maybe you can make something that simple sound good. Just an idea.
 
hmmm you could try the what compressor which is an opto compressor maybe something you may not want for guitar?

As for eq's I think the pultec is great on guitar and I think you could squeeze it in a 1 space rack unit. Maybe mount the tubes on the back?
 
the PRR varimu is indeed simple and cheap to build. it thumps pretty hard too when you get it pumping but it leaves the highs nice and clear! You'd be very happy with it.
 
[quote author="Svart"]the PRR varimu is indeed simple and cheap to build. it thumps pretty hard too when you get it pumping but it leaves the highs nice and clear! You'd be very happy with it.[/quote]

Oh thanks! I have 12au7 and 6bc7 tubes around already too. I'd just need the op-amps and the 100V PS and it would fit in the right sideof the case I have around. I prolly have xformetrs in my jusk box for the pre and the vari-mu.

Arg I dunno about building a whole freakin pe1q for a guitar box. :(
I wish there was some simple 4 band circuit I could put in between mic pre stages.

Kiira
 
[quote author="rafafredd"]You could build a normal mic preamp with a sidechain driving an opto in paralell to the gain feedback-resistor. You could include the EQ circuit in the feedback loop also.[/quote]

um... huh?


That would be the simplest channelstrip ever, I think... And maybe you can make something that simple sound good. Just an idea.

LOL I seriously doubt if I'm the one to take on this challenge. :)

I was just thinking that if there is the EQ circuit after the input stage... hmmmm like use something like a 6sn7 cathode follower then put a bypass switch to cut in the EQ and use the output stage for make up gain. Maybe use ome 5687s with a 6sn7 cathode follower feeding a 1:1 output trafo. Then put the vari-mu next.

Kiira
 
[quote author="pucho812"]what about a tone circuit as opposed to EQ?[/quote]

well that would be super easy a regular old fender high, mid low tone stack? I wanted something a bit more zoot . like maybe 3 or 4 bands.

Kiira
 
You could put in a Pultec EQP or MEQ filter section without the bell shaped curves. This would simplify matters a great deal, and you would not have to source an inductor. You might be able to customize the cap values for guitar.
cj
 
[quote author="CJ"]You could put in a Pultec EQP or MEQ filter section without the bell shaped curves. This would simplify matters a great deal, and you would not have to source an inductor. You might be able to customize the cap values for guitar.
cj[/quote]

I'll try that then. That seems lke the best idea. Where do you thonk I should put the EQ?

Kiira
 
Just to throw another circuit into the pot (pardon the expression): this is a shelving EQ from QUAD, mid-1950s vintage. It's all passive, and needs about 20dB of makeup gain (*SN7 or 12AU7). The pots are log taper.

quadtone.gif


You could combine that with a couple of bands of peak/dip if you so desired.

Peace,
Paul
 
Hey thank you Paul but I really want this thing to have some serious EQ. I guess I will just have to bite the bulletand build a Pultec into the box. I have one of Gustavs boards on the way and I can use that... arg.... 12 knobs on a 1U box.
 
noone has tried it yet (AFAIK), but PRR mentioned using high ratio stepup transformers to convert the line input of his vari-mu design into a mic pre. i would imagine that some inexpensive 1:6.45 + 6.45 OEPs could work there. if all that it takes is really the transformer swap, then it would be pretty easy. search the PRR varimu threads (for posts with PRR as author) for more info.

also, you could insert an EQ section in after the "interstage" transformer and before the 5532 output driver (or whatever tube circuit you might use instead) on the PRR varimu. give the NYD EQ circuit a look.

ed
 
so if you are going to insert a "serious" eq into the circuit, why not go gyrator for a couple fully parametric bands? the EQ in my console is hi shelf, 2 overlapping parametric mids and low shelf.. works great! you'll need a couple quad opamps and a couple dual opamps for summing.. could be real easy and small if you do SMD.. you could still fit it in the varimu circuit.. or before it so you don't compress audio that you'll be cutting anyway.
 
[quote author="Svart"]so if you are going to insert a "serious" eq into the circuit, why not go gyrator for a couple fully parametric bands? the EQ in my console is hi shelf, 2 overlapping parametric mids and low shelf.. works great! you'll need a couple quad opamps and a couple dual opamps for summing.. could be real easy and small if you do SMD.. you could still fit it in the varimu circuit.. or before it so you don't compress audio that you'll be cutting anyway.[/quote]

What's a gyrator? Is there a schematic aroun? I suspect people are suffering from the misconception that I actually know anything. I don't. I solder real good.


Kiira
 
:green: sorry, a gyrator is an opamp(or other active element) based circuit that emulates the way an inductor works. you can use these to create circuits that act like L/C EQ circuits without the expensive or special inductors.

I have a schematic floating around, I'll have to dig it up and post it.

I've been thinking of laying out a 4 band parametric(frequency, gain and bandwidth adjustments) EQ based on parts of this circuit for a while, maybe it's time to do it..


:thumb:
 

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