Aikido Preamp Bass & Treble Controls - Page 2 - diyAudio
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Old 21st February 2013, 08:53 PM   #11
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kouiky,
I bought a board recently which used the following active Baxandall circuit:
Click the image to open in full size.

The tubes in mine are 6N2P.

Also, there's this:
ebay tube preamp with tone controls

The construction of my board is very good, considering the source (China).
(I only say that after a series of disappointments with Chinese-made products being poor quality).

I haven't measured the boost/cut etc but it does give very good control over tone and plenty of gain. I'm sure you could tweak the circuit if you want.
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Old 21st February 2013, 10:07 PM   #12
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by kouiky View Post
Thank you for your reply tinitus, the center points on stock preamp's tone controls has always been a point of contention for me because they modeify too much of the midrange, and modelling them myself has helped considerably. They're not overly complex and it leaves one wondering why designers do not offer more options for slope points on their preamps. I would find it strange that glassware withholds specifications on their products, the absence of which leaves many questions unanswered by the audio industry is rife with this.

Hello Loren42, thank you very much. Do you know of any other preamp boards available with provisions for tone control circuits? Can you recommend a good source for standard width cases/chassis or at least a faceplate with several rotary controls? Thank you kindly.
Well, I just rolled my own based on the following design. Most of the circuit is on a PCB, but the tone control pots and associated components are hand wired to the pots.

I offer no guarantees how this will work, so proceed at your own risk.

The tone pots on the diagram are resistors R14 & R15 for the bass pot (100K) and R16 & R17 for the treble (100K). I set the values of those resistors to demonstrate full boost for each.

This circuit has stunted boost and cut. You get a little more than 9 dB with the bass and about 6 dB with the treble.

Note that the midrange is fairly wide, with the bass boost starting at about 700 Hz. The +3dB point is about 270 Hz.

Click the image to open in full size.

I have the circuit built, but not yet fired it up. I still have a few more wires to connect, then I will take it to my friends house as he has all the test equipment. That may be a few more weeks.

Click the image to open in full size.

The board actually has provisions to add an extra stage on the front end for about 6 dB of gain, but I can cut a jumper and pull some parts to bypass that stage. I think that is the way I will run mine as I do not need that 6 dB of gain.

All the caps are mounted on the back side of the board to get away from the heat. Filaments are DC. Board size is 3.8" by 2.5" and it contains a single channel.

Lastly, I had a different project that had some board space on it that was unused so I decided to try Rod Elliot's preamp with tone controls using solid state op amps (OPA2134). I used some surface mount components (all resistors) and packed two channels int a board 2.5" long and .9" wide!

Click the image to open in full size.

Since I already had the parts in my parts drawer and the board comes free, why not? I may build this up and test/compare it to the tube version. I am sure the solid state version will be quieter and lower distortion, so I may end up using that as my final preamp.

I do not know where to buy a chassis. I have a mill and lathe in my garage, along with all kinds of scrap aluminum. However, the box took a lot of hours to make! I need to get the parts black anodized after I test the circuits.

I built it so it can house high voltage, filament, and a 15 VDC supplies all at the same time. It is more of a test bed than a dedicated design.

The large gray box in the rear has a set of relays to select the inputs and ground the unused inputs.

It also has line out and balanced outputs.

I hope this gives you some ideas for your own design.

Click the image to open in full size.
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Old 21st February 2013, 10:52 PM   #13
tinitus is offline tinitus  Europe
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I had forgot about this 'tutorial' site
there is a quite good section with tone controls, half way down

Transistor tutorial Audio Amplifiers, Part 5
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Old 28th March 2013, 06:59 PM   #14
kouiky is offline kouiky  United States
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Loren42, any update on a Solid vs Hollow state comparison? Thank you, as always.
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Old 28th March 2013, 08:00 PM   #15
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Quote:
Originally Posted by Loren42 View Post
Have a look at Rod Elliot's Preamp 97 for a comparison. The cost and performance should be a good bit better.

Rod Elliot Hi-Fi Preamp

I would substitute something like the OPA134 or OPA2134 for the TL072 chips.
I like that circuit. And I like that the tone "defeat" only limits the tone controls effects to a few dB. It's very practical, purists be damned.

Is there any reason you couldn't use a 5532 instead? Since it subjectively "sounds better" it might be a cheap, if marginal, upgrade?
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Old 28th March 2013, 08:02 PM   #16
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Quote:
Originally Posted by tinitus View Post
I had forgot about this 'tutorial' site
there is a quite good section with tone controls, half way down

Transistor tutorial Audio Amplifiers, Part 5
I fail to see the advantage in those circuits over Elliott's design. And I think that the passive circuit uses audio taper pots, which would make the circuit even more imprecise.
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Old 28th March 2013, 09:18 PM   #17
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by Fast Eddie D View Post
I like that circuit. And I like that the tone "defeat" only limits the tone controls effects to a few dB. It's very practical, purists be damned.

Is there any reason you couldn't use a 5532 instead? Since it subjectively "sounds better" it might be a cheap, if marginal, upgrade?
I have not built my boards yet. I have a honey-do list to finish the entertainment cabinet first.
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Old 29th March 2013, 01:00 PM   #18
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by Fast Eddie D View Post
I like that circuit. And I like that the tone "defeat" only limits the tone controls effects to a few dB. It's very practical, purists be damned.

Is there any reason you couldn't use a 5532 instead? Since it subjectively "sounds better" it might be a cheap, if marginal, upgrade?
No reason not to use a NE5532.

I actually bought LM4562 chips for mine.
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