NEW Bass Pre-amp Build HPF/Compressor

[Wardy]

Hi, first a big thanks to all here for your wisdom.
I'm new to this amp building but learning fast. This forum has been a great help. Many thanks especially to Sluckey for your Tremonator circuit!

I need some help with my next project. It will be an all tube Bass Guitar preamp. I'll power it with a 12dc wall wart and boost the supply to provide 250v plate.
Required specs.
Vol, bass,mid,treble, master. Then I want a tube compressor. and a tube variable high pass filter. plus a DI out.

I previously built  lov Sugarbox from Tubetown Germany and it sounds pretty good so this can form the basis for the new preamp. I need to find satisfactory compressor and HPF. So I have found a tube compressor circuit but could do with an expert taking a look this. I've attached here. I am strugging to find a variable HPF. need something like a FDeck but I want a tube version. Any thoughts?

[Merlin]

A tube compressor is not a trivial project, it's likely to at least double the size of your circuit! Unless you actually mean you want an opto (LDR) compressor, which can be slightly simpler?
I'm not sure what you want from the HPF. Is a simple CR filter using a pot not satisfactory?

[Wardy]

Thanks Merlin,

I understand it will get bigger. thats not an issue as I will fit into a chassis about 12" x 8 x 2 so can get quite a bit in there. I've attached a circuit I found of a compressor but not sure how it might perform. Would appreciate your critique of it and whether you think it would be worth a try. Re the hpf what I want to achieve is a hpf that is variable between about 30hz and 120hz. i use this if I'm playing in a room with reflections that cause low end rumble to clean things up and its really handy. I use a broughton audio stomp unit at the moment and an empress compressor but would like to have basic compression and the hpf in the preamp to get rid of the pedals. I could use solid state and there are loads of circuits out there but would prefer tube if poss.

[Merlin]

That's really an AGC (aka AVC) circuit. It will compress a bit, but it will be dirty (distorted). That might be OK for your application, but you're in experimental territory. Few people have made successful DIY tube compressors, and mostly what they end up with are FX boxes that kinda, sorta, do some compression. Or maybe I'm just a compressor snob.

[_Steve]

In the 2-valve compressor schematic posted, how does the V1b triode function?

If looks as though the anode is AC-connected to the cathode of V1a but has no DC HT?

Also it's cathode is bypassed so wouldn't its output signal connected to the grid of V2b be AC-grounded?

[Wardy]

Thanks Merlin,

I guess thats not goona be what I want then, I have a schematic of a single channel LA2A but it is still more complex than I need, it has sidechain etc and I don't need this.  Is the  simplified version of an LA2A and would this be a better option?

Steve I don't know the answers to your questions I am too new to this, I'm sure Merlin would though.

[Merlin]

In the 2-valve compressor schematic posted, how does the V1b triode function?
If looks as though the anode is AC-connected to the cathode of V1a but has no DC HT?
Also it's cathode is bypassed so wouldn't its output signal connected to the grid of V2b be AC-grounded?

If you think you don't understand a circuit, try re-drawing it.

• The first triode is a cathode follower buffer.
• The second triode is being used as a diode to half-wave rectify the audio, to produce the control voltage, smoothed by the .01 timing cap.
• Third triode is another cathode follower with a variable resistor to (in theory) vary how much control voltage reaches the fourth triode.
• Fourth triode is supposed to behave like a variable resistance, forming the lower arm of a divider that ultimately compresses the audio. As the control voltage rises, the internal resistance of this triode falls, which should compress the audio.

However, it all looks a bit funky. The grid of the 'diode' is shown unconnected, which I suspect is a typo (I would expect it to be strapped to the anode?). The 'ratio' pot looks like it will cook the triode if it is turned down to zero, so I'm thinking it was supposed to be drawn after the 15k resistor instead, or something? Various cap polarities look backwards to me. There will be a lot of control voltage feedthough, which is characteristic of these AGC circuits. Like I say, if it compresses at all, it's a dirty compressor.

[Merlin]

In most, simple AGC circuits like this one, the DC control voltage gets added to the audio signal, which creates a 'thump' every time the compressor kicks in (in the same way that some valve tremolos thump). They were intended for radio receivers rather than quality audio production. Getting rid of the thump is most of the reason why 'proper' valve compressors use push-pull circuits, with transformers. However, this paper goes on to discuss some clever and simple thump-less circuit ideas, but they would need development. I can share the full paper if anyone is interested.
From: Audio Automatic Volume Control Systems, JAES, October 1953.

[Willabe]

I can share the full paper if anyone is interested.

Yes please.

[Merlin]

Yes please.

https://www.dropbox.com/scl/fi/wf3jf7msfrqt5h5kw1u6b/1953_AGC_Systems_Roberts.pdf?rlkey=cxjh10nzkm28v0pfw8tndp5ms&st=ui0t9zvt&dl=0

[kagliostro]

As this is is the right thread to post it

I post also here the schematic from my archive that I posted on the Bass Preamp thread where Wardy told he wants to build a Preamp for Bass with Compressor

I recovered this schematic many years ago and it was published by an East Europe guy who build a Tube Bass Amp with it inside

COMPRESSOR - EXPANDER with ECH84
If diode D is inverted, it functions as an expander. The top potentiometer adjusts the gain.
The bottom potentiometer adjusts the compression.
The 100-500k trimmer adjusts the delay



Franco

[Wardy]

OK so here is another circuit that has emerged. Any thoughts on whether its worth a go?

many thanks

Steve

[Merlin]

That looks very good!

[Wardy]

Thanks Merlin,

Looks like a good candidate then. Thats 4 tubes needed so far, I think I can fit 5 in the chassis I have. I'm need to decide what variable HPF to use and a DI out. Then I can get building.

best

Steve

[Merlin]

I came across this preamp with tone controls and compression & expansion, and thought it might interest you, as it sounds very similar to what you're actually trying to build! The compressor is the thumpy sort, but at least it's simple, and I was tickled by the way it does compression or expansion, depending on the direction you turn the knob.
HI-FI CONTROL AMPLIFIER WITH "EXPRESSION, Radio & TV News, August 1955, Page 50:
https://www.worldradiohistory.com/Archive-Radio-News/50s/Radio-News-1955-08-R.pdf

[Wardy]

Sounds fascinating!

I've already committed to the instacomp circuit and have most of the parts to hand, a revised version attached. I think that It'll definitely go on the list of future projects though. I have been reading your work a lot and it is most helpful, although I'm on a steep learning curve. Another question I have is how to optimise the sugarbox preamp to operate at a B+ of about 300v rather than the 60v. So far have I have used the load line calcs for 12au7 on ampbooks to adjust the value of the cathode resistors to aim for for a midway point to enable the most headroom but not sure if this is the right approach or if I have made correct assumptions.

best

Steve

[Merlin]

Another question I have is how to optimise the sugarbox preamp to operate at a B+ of about 300v rather than the 60v. So far have I have used the load line calcs for 12au7 on ampbooks to adjust the value of the cathode resistors

In a guitar preamp, cathode bias is normally trimmed for tone, rather than headroom. Cathode bias is also highly self-scaling with B+. So basically I would not make any changes at all, just run it at 300V and see how you like it. Then tweak the bias if you're not satisfied.

[Wardy]

Many Thanks Merlin,

Looks like I need to reach for the desoldering braid then..............

Regarding the varimu compressor circuit, I have most of it together on a prototype board, its not pretty but I should be correct to to circuit. please excuse my lack of knowledge but I don't understand the difference between the power in supplies B1 and B2. B2 is marked as 250v but nothing on B2. not sue what I should be supplying here? any willing helpers?

many thanks

Steve

[Hermanni]

B2 is supplied through R13. No need to connect any other voltages to B2.

[Wardy]

H Hermanni,

many thanks for that info. I thought that might be the case but wasn't sure. that was very helpful.

I have the compressor more or less built just beed to put all the different boards in the chassis now and wire up. In the mean time I also found the attached BTE varicomp in the schematic library of this forum. Does anyone know anything about this? i can't see any other info about it online.

[Hermanni]

I don't have any info on the VARICOMP except by looking at the schematic it's a fairly simple OPTO compressor. It uses two photoresistor when it could have been done with just one. Two were probably used to get the wanted response time.

[Wardy]

Thanks Hermanni,

I was thinking it would be nice to have some kind of as simple as possible gain reduction metering on the vari-mu compressor once I get it all together. I have found a circuit which is more complex than I would like but not sure where to insert it, any ideas?

best

Steve

[Hermanni]

If you want something simple just use one led for gain reduction idicator. Connect IN to instcomp CV.

[Wardy]

Hermanni

Thankyou