Active preamp, no EQ

[Rob22315]

I've seen that ckt before...
here's a link that goes thru the numbers that you need.

Elliott Sound Products is another website I frequent. There are a lot of audio circuits with really good explanations. Somewhere on that site I'm sure there is an explanation of the Active Baxandall Tone Ctrl Ckt.

Great, thanks for the link. Any problems with that circuit? Is there another you prefer? I'll check out the other website you mentioned. As near as I can tell, the original Baxandall circuit is 2 band. I'm looking for three. I'll see if there's a modded design that gives me three band.

 

[rumblinbass]

Great, thanks for the link. Any problems with that circuit? Is there another you prefer? I'll check out the other website you mentioned. As near as I can tell, the original Baxandall circuit is 2 band. I'm looking for three. I'll see if there's a modded design that gives me three band.

a 3-band is pretty easy - just combine the high and low frequency feedback paths.
Baxandall tone control is about as generic as it gets. if you get creative you can get a really nice tone control ckt. Pick the right frequencies and you can't go wrong.

 

[cassius987]

If you want to try the JFET route; you can get a circuit board (and schematic) for the Stratoblaster here:

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And an op amp buffer:

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What does the "Boost" pot of the Stratoblaster do? Can it just be bypassed with a jumper from G3 to G2?

 

[Rob22315]

I've seen that ckt before...
here's a link that goes thru the numbers that you need.

Elliott Sound Products is another website I frequent. There are a lot of audio circuits with really good explanations. Somewhere on that site I'm sure there is an explanation of the Active Baxandall Tone Ctrl Ckt.

After doing some reading you suggested, I have to ask - are you trying to trick me? As near as I can tell, both the linked schematic and the active baxandall are all the same thing - exactly the same layout for bass and treble control (values differ) with a combined filter for the mid control. I also read the 'how to' reference you provided. There's plenty there to allow me to work all this out on my own.

If the net NTMB schematic is correct, it uses almost the same design except with L and C for the mids, not two Cs. There are a couple of resistors in the NTMB I can't quite figure out.

 

[rumblinbass]

After doing some reading you suggested, I have to ask - are you trying to trick me? As near as I can tell, both the linked schematic and the active baxandall are all the same thing - exactly the same layout for bass and treble control (values differ) with a combined filter for the mid control. I also read the 'how to' reference you provided. There's plenty there to allow me to work all this out on my own.

If the net NTMB schematic is correct, it uses almost the same design except with L and C for the mids, not two Cs. There are a couple of resistors in the NTMB I can't quite figure out.

not quote sure I understand...trick you?
that schematic you linked is an active baxandall ckt. there are lots of different flavors but all do the same thing.
the link I posted provides what you asked - an explanation of the ckt and how to calculate values and frequencies.
you can use the same exact ckt for low and high frequency filters - a combined ckt - if you want low and high frequency roll-offs. For example, limiting the passband frequency response to 20Hz - 20kHz.
not sure what to tell you - I provided a link that explained the ckt and how to crunch numbers. I thought that is what you asked for.

Not sure what to tell you about the NTMB ckt. I haven't seen it. Like I said in an earlier post - you can get pretty creative with this ckt with some pretty interesting results.

If you are curious and want to play around with values I suggest you get a SPICE program. LTSPICE is available for free and pretty accurate.

 

[SGD Lutherie]

What does the "Boost" pot of the Stratoblaster do? Can it just be bypassed with a jumper from G3 to G2?

On the real Stratoblaser the gain control is a trim pot and is on the circuit board. At minimum the buffer should be at unity gain. It's a 50K trim pot, so a 50K fixed resistor should give you unity gain. A jumper would be full gain, which is enough to drive a power amp.

I build my buffers with trim pots, as in the photo I posted earlier. That's essentially the same circuit. You should increase the output coupling cap to 5µF for better bass response.

 

[Rob22315]

not quote sure I understand...trick you?

If you are curious and want to play around with values I suggest you get a SPICE program. LTSPICE is available for free and pretty accurate.

Thanks for the tip about LTSPICE, I'll try it. I took a quick look at LTSPICE, it looks like it only does passive components?

I interpreted the wording in one of your posts to say all these circuits were different and you preferred the active bax over the other one. I figured you were having a little fun with me since it's been a while (like 30 years) since I've dealt with real EE.

 

[cassius987]

On the real Stratoblaser the gain control is a trim pot and is on the circuit board. At minimum the buffer should be at unity gain. It's a 50K trim pot, so a 50K fixed resistor should give you unity gain. A jumper would be full gain, which is enough to drive a power amp.

I bet it's tempting for some to just go with full gain, but is there a trade off? Like... shorter battery life, or problems "taming" the output?

 

[rumblinbass]

Thanks for the tip about LTSPICE, I'll try it. I took a quick look at LTSPICE, it looks like it only does passive components?

I interpreted the wording in one of your posts to say all these circuits were different and you preferred the active bax over the other one. I figured you were having a little fun with me since it's been a while (like 30 years) since I've dealt with real EE.

no it has a lot of active parts...a lot.
it is a bit tricky to navigate and find your way around but once you do it is a very useful tool.

 

[SGD Lutherie]

I bet it's tempting for some to just go with full gain, but is there a trade off? Like... shorter battery life, or problems "taming" the output?

It's too loud! That's about it. But it's nice part way up.

The original circuit was intended to be installed on a Strat replacement output jack plate, and was intended to be cranked up. That's the guitar tone you hear on the Fleetwood Mac albums with Lindsey Buckingham.

 

[Rob22315]

no it has a lot of active parts...a lot.
it is a bit tricky to navigate and find your way around but once you do it is a very useful tool.

Yes, I found the opamp components. The amount of other choices is mind-boggling.



I also found Baxandall's original paper from 1952. It lays out how to calculate the various values for a 2 band design pretty well although his design uses tubes. He did use the same 'virtual ground' concept used in designing opamp circuits. Apparently, that technique was popularized just a couple of years before the article was published.


I found three different approaches to implement the three band circuit - all Baxandall, all parametric or a hybrid. There are plusses and minuses to all of them. The primary problem with a 3 band Baxandall is that the frequency bands overlap which means the controls interact (this doesn't occur in his original 2 band design). The problem with the parametric (separate active controls for each band) is that it uses more opamps than necessary which isn't a huge cost problem but I'd like to keep the number of opamps to four so I can use a quad chip. I'm looking at a hybrid design - a pickup buffer/blend, a 2 band active baxandall, an active mid cut/boost, then a final buffer stage. I'm not yet sure where I'll put the volume - either at the buffer/blend or at the output buffer. The 2 band wants to see a high output impedance from the stage before so I have to factor that into my choice of volume control location.



Once I figure out LTSPICE, I'll see how each of the circuits do.

 

[rumblinbass]

Yes, I found the opamp components. The amount of other choices is mind-boggling.



I also found Baxandall's original paper from 1952. It lays out how to calculate the various values for a 2 band design pretty well although his design uses tubes. He did use the same 'virtual ground' concept used in designing opamp circuits. Apparently, that technique was popularized just a couple of years before the article was published.


I found three different approaches to implement the three band circuit - all Baxandall, all parametric or a hybrid. There are plusses and minuses to all of them. The primary problem with a 3 band Baxandall is that the frequency bands overlap which means the controls interact (this doesn't occur in his original 2 band design). The problem with the parametric (separate active controls for each band) is that it uses more opamps than necessary which isn't a huge cost problem but I'd like to keep the number of opamps to four so I can use a quad chip. I'm looking at a hybrid design - a pickup buffer/blend, a 2 band active baxandall, an active mid cut/boost, then a final buffer stage. I'm not yet sure where I'll put the volume - either at the buffer/blend or at the output buffer. The 2 band wants to see a high output impedance from the stage before so I have to factor that into my choice of volume control location.



Once I figure out LTSPICE, I'll see how each of the circuits do.

As I said earlier, there are many different flavors and this topology has been implemented is all kinds of ckts - tubes, discrete 2 and 3 BJT ckts, op amps, and passive.

If you plan on using an op amp you shouldn't have to worry about source and load impedances...op amps make things easy that way.

If you choose your frequencies right interactions can be minimized. I would start with low and high frequencies get those where you like them, then start playing with mid frequencies. From what I understand a somewhat popular low and high frequency selection is 80Hz and 4kHz. Now, how you want to define your filters is up to you.

As far as the Volume pot. That really should be the last stage. If you want to put a buffer following the pot - that's ok, but I would not follow it with a gain stage. Yes, I do consider tone control a gain stage. There is boost, it is frequency limited, but it is gain.

 

[Rob22315]

If you plan on using an op amp you shouldn't have to worry about source and load impedances...op amps make things easy that way.

If you choose your frequencies right interactions can be minimized. I would start with low and high frequencies get those where you like them, then start playing with mid frequencies. From what I understand a somewhat popular low and high frequency selection is 80Hz and 4kHz. Now, how you want to define your filters is up to you.

As far as the Volume pot. That really should be the last stage. If you want to put a buffer following the pot - that's ok, but I would not follow it with a gain stage. Yes, I do consider tone control a gain stage. There is boost, it is frequency limited, but it is gain.

The source doesn't feed directly into an opamp input, it feed the RC network in the feedback loop. Too high an internal source resistance will throw off the Rin.


I'd like a lower bass frequency, probably 50. I'll put the mids at 500-600. 4K for treble should be fine.


Thanks for the advice on where to put the vol pot.

 

[SGD Lutherie]

I like 80Hz for bass controls. I used to think something lower like 50Hz would be the way to go. Then I noticed that I liked the EQ on my Roland mixer, and the bass is at 99Hz. I played around with it, and 75Hz-85Hz is very punchy and doesn't get too boomy.

I think I like 5 kHz better than 4 for treble. Maybe even 7 or 8 kHz.

 

[rumblinbass]

The source doesn't feed directly into an opamp input, it feed the RC network in the feedback loop. Too high an internal source resistance will throw off the Rin.


I'd like a lower bass frequency, probably 50. I'll put the mids at 500-600. 4K for treble should be fine.


Thanks for the advice on where to put the vol pot.

if the tone control is fed directly from an op amp it should be fine.

I like anything below 80Hz. Too far above that and it gets nasty to me. I have found that the higher the frequency the less boost I like.

One of my bench marks is an Aguilar DB924 I used to own. I loved the tone control on that. It was just a 2-band tone control, but sounded so good. It was very powerful without being too drastic. I can't remember the high frequency but the low frequency was 40Hz.

Now that I have said that, another important aspect of a tone control ckt is the shape of the curve. That is what makes one filter, tone control, eq, etc. different from one another.

 

[Rob22315]

I like 80Hz for bass controls. I used to think something lower like 50Hz would be the way to go. Then I noticed that I liked the EQ on my Roland mixer, and the bass is at 99Hz. I played around with it, and 75Hz-85Hz is very punchy and doesn't get too boomy.

I think I like 5 kHz better than 4 for treble. Maybe even 7 or 8 kHz.

The sound guy wasn't around last night. I played around some more with the soundboard and got the treble added back into the bass channel. I'll have to play around with the treble more now that I can hear more of the complete tone of the guitar.


I noticed last night that we've got a fairly complete set of both stand-alone preamps and preamp settings on the sound board. Experimenting with these should help give me a better feel for what I want.

 

[Rob22315]

Now that I have said that, another important aspect of a tone control ckt is the shape of the curve. That is what makes one filter, tone control, eq, etc. different from one another.

You spoke highly of the baxandall curve shape - it doesn't look like your standard low pass/high pass filter shape. I played around with the Duncan tone calculator for a little while and didn't see much that was better than that.
What do you recommend for mids? I know I can shape the filter Q and I could go with something second order or third order to get steeper curves.

 

[rumblinbass]

You spoke highly of the baxandall curve shape - it doesn't look like your standard low pass/high pass filter shape. I played around with the Duncan tone calculator for a little while and didn't see much that was better than that.
What do you recommend for mids? I know I can shape the filter Q and I could go with something second order or third order to get steeper curves.

I believe the Duncan Tone Calculator is all passive networks. If I am not mistaken you are using an active ckt.

The Baxandall curve shape is pretty unique, but very familiar. This ckt is (was) used everywhere. The shape of the curve can be manipulated with some added components.

For the mids I recommend the Baxandall ckt. You are now familiar with the ckt and should be somewhat easy for you to zero in on a frequency you like. Just be careful about finding that frequency on another piece of gear. It's a good idea, just keep it in perspective and may sound different than what you derive. This is where the shape of the curve comes in. I wouldn't really worry about Q for this application.

This is an onboard bass preamp. Keep it simple. Leave all the surgical eq'ing/tone control to your amplifier or preamp.

 

[Rob22315]

I believe the Duncan Tone Calculator is all passive networks. If I am not mistaken you are using an active ckt.

Yes but it's the same circuit just placed in a feedback loop. I can still get a good feel for the interaction of the various components and how they impact response.

 

[anonymous101511]

They wouldn't fit my bass, first of all.

Also, not a big fan of their tone. Love them for electric guitars, not so much for bass.

just need to stuff the battery in there, easy with the lo- profile pots they give you.