Aikido Preamp Bass & Treble Controls

[kouiky]

Hello DIYers,
I was looking to build a line-stage preamp and was considering the Aikido 9-pin or 5687 from Glassware Audio. Something I miss having at my disposal is a preamp with tone controls, particularly a bass control that operates more at the extreme lows rather than muddying up the midrange. From what I understand, the volume pot would feed the Aikido gain stages board, then feed the tone control circuit, and finally feed a buffer output stage. I wanted to consult other hollow state builders to get some advice to avoid noise, hum, instability or what other issues may or may not present themselves in a build like this, as I was a solid state guy. What I would need is about +8dB bass boost at max at 10--30Hz with little effect above 100-200Hz, and -6dB treble cut max at 12kHz with little effect below 2kHz. Thank you for your consideration.

 

[Loren42]

I think the idea of using tubes and tone controls is a very difficult adventure. To do this requires stages with enough gain to compensate for the loss of gain in the tone circuit and then adequate drive at a low enough impedance to drive what you have as a power amp (which is another bag or worms - trust me).

To do all this requires a lot of tubes and the more tubes you throw to the circuit, the more noise, hum, and distortion jump on the band wagon. Designing with tubes is also far more technically challenging because there are far more variables that need to be done right, not just adequately.

While you may be able to use the Aikido line amp as one of those stages, you will still need additional gain stages to make up for the loss of the signal as it passes through the tone network. Figure on at least 15 dB of gain if not more, depending on how much compensation you want in the tone circuit.

While doing all of this you lose fidelity in the signal chain. Ultimately, you will compare your results with a good solid state preamp as a benchmark, with the idea of not being able to tell the difference in a blind test between tubes and solid state (i.e., OPA134 or equivalent). If you get that, then you really have something worth while - while the tubes last (remember, they change with age). Also, if you get less than 0.1 % distortion that would be amazing, but chances are you are looking at closer to 1% distortion. Even a bad solid state op amp design can do that.

However, that begs the question of why would you go through all of those hoops to get equivalent performance when you can just use op amps and be done with it for far less cost, less heat, lower complexity, and far better longevity and reliability?

On the other hand, if you are doing this for the love of a challenge, then why start with someone else's design (i.e., Aikido)? Why not just become an expert in valve design so you can turn your own?

 

[tinitus]

What I would need is about +8dB bass boost at max at 10--30Hz

that one you build into you woofer amp, active xo, or sub

and -6dB treble cut max at 12kHz with little effect below 2kHz.

relatively simple, its a resistor and cap mounted on input of your tweeter amp

its is a bass Eq, and treble cut, not tone controls

but as said, you don't need tubes for that
actually, I'm not really sure tubes are the best for bass Eq

 

[kouiky]

Thank you for the replies gentleman.
Hello tinitus, this is a preamp for passive speaker systems powered by one stereo amplifier. I have passive speakers that I like and I listen at low levels but more or less wanted the ability to change the tonal balance at a moments notice, depending on what I was listening to. Otherwise, I would never need bass gain or treble control.

Thanks Loren, I really took the consideration into hollow state for odd reasons. I actually like the low order harmonics that tubes sometimes provide in the midrange and bass regions on headphones. I was using a 6N3 tube headphone amplifier and digital eq via PC and it was a good experience. I have also listened to several tube preamps at a friend's residence and one sounded a bit light but crystal clear at all frequencies, until a component had a bad day and then it sounded rather lifeless in all regards. I cannot recall what the builder said went bad. I chose the Aikido as a start point to ask because it appeared to have a good reputation for sound and it offers a straight forward build with the cumulative design work finished and PCBs ready to populate. The Aikidos are stereo and supposed to offer about 20dB of gain, and the attenuation through the tone control circuitry could be at about 15dB and feed into a buffer from the same maker, to drive the power amplifier, if you think that could work. At this time, I do not really need a lot of gain, as I do not listen loud and currently use a passive attenuator. If you think this simply would not work and a solid state pre with tone controls would yield better sonic results, I appreciate your opinion and experience.

 

[Loren42]

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.

 

[kouiky]

Thank you very much Loren42. I wonder why Rod designed the output buffer to operate inverted rather than in phase? Along with the OPA2134 I would feel inclined to try the LME49720, too just to give it a try.

 

[tinitus]

I have passive speakers that I like and I listen at low levels but more or less wanted the ability to change the tonal balance at a moments notice, depending on what I was listening to. Otherwise, I would never need bass gain or treble control.

ok, I understand now, fair enough

you could look at a loudness control for the low part
have even seen one that was built into the volume pot
and maybe a Quad style Baxandall tilt control

normally they might be considered worthless
but the sweet thing is, being DIY we can tailor it to suit our needs much better
and all the info is there now, and software calculators etc

or maybe just the Baxandall tone control will be all you need
and that one can be done perfectly and fully passive between your input gain tube and output buffer
but with the Aikido...I have no idea, sorry

 

[Loren42]

Thank you very much Loren42. I wonder why Rod designed the output buffer to operate inverted rather than in phase? Along with the OPA2134 I would feel inclined to try the LME49720, too just to give it a try.

The last stage of the tone controls also inverts, so the final stage inversion just brings everything back into phase.

Everything I read about the different chips leads me to believe that the difference in op amps is too little to be audible.

I know people can swear they hear or think they hear a difference, but until you do a real blind or even a double blind test, I just take those statements as subjective chatter.

The other thing is that different chips need slightly different circuits to sound their best, so simply transposing another chip may actually and more often than not, lead to a degradation in sound.

That being said, the TL072 is getting a little old.

 

[Seth1334]

That being said, the TL072 is getting a little old.

LOL
So is the EF86 but people still use them to great advantage.

 

[kouiky]

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.

 

[Seth1334]

kouiky,
I bought a board recently which used the following active Baxandall circuit:
[IMGHTTPDEAD]http://sethasaurus.x10.mx/tubeaudio/Tone-Control-Baxandall.jpg[/IMGHTTPDEAD]

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.

 

[Loren42]

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.

[IMGHTTPDEAD]http://www.mdbq.net/diyaudio/tonecontrols.jpg[/IMGHTTPDEAD]

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.

[IMGHTTPDEAD]http://www.mdbq.net/diyaudio/LineAmp.jpg[/IMGHTTPDEAD]

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!

[IMGHTTPDEAD]http://www.mdbq.net/diyaudio/pre01.jpg[/IMGHTTPDEAD]

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.

[IMGHTTPDEAD]http://www.mdbq.net/diyaudio/prechassis.jpg[/IMGHTTPDEAD]

 

[tinitus]

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

 

[kouiky]

Loren42, any update on a Solid vs Hollow state comparison? Thank you, as always.

 

[Fast Eddie D]

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?

 

[Fast Eddie D]

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.

 

[Loren42]

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.

 

[Loren42]

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.