Treble Boosters what matters?

[mac]

Just from the standpoint of selling a treble booster, I prefer the use of high input impedance and frequency determination being safely protected by a high-impedance input and low-impedance output.

I built a "treble booster" that has a Multiple Feedback Band Pass followed by a direct coupled transistor biased a Vc=7v, Vb=4.5v.
The center frequency can easily be adjusted with a pot. In a Rangemaster this is done by changing input caps 
The transistor does the soft clipping.

I guess that biasing the opamp at 7v with a bit of gain can soft clip without using a transistor...

mac

[Wavelength]

All,

Ok so last week I found a Beano Boost on my shelf, I forgot I had one it's a Mullard CV5713/OC71. Mike does such great work and as we can see from the Beano Boost post going on now as well that Mike does and inverted PNP design. By inverted I mean no battery so the emitter side of the circuit goes north to 9V and the the output pot is grounded.

When I did my testing earlier this week I did not realize my Prism was pulling down the output and changing the results. So now the output is going to a boss tuner in buffered mode so ~500-600K input z.

The test set up is like this:

MacBook Pro running Signal Scope Pro sample rate 88,2KHz DragonFly Black v1.5 USB DAC (my design, 0.4ohms output z) 10K PEC pot->Unit Under Test->Output. I did listening tests yesterday with varied T type and Humbuckers and both the Beano and my Nitrous T sounded pretty much the same on the T types and the Beano a little louder and bolder on the Humbucker. Anyway gain on the Beano was much higher than Nitrous T. Beano at 10 oclock, NT at 1 oclock for the same input output gain ~=4 or 12dB. So the output of the DAC was set to 0.119Vac rms, all the meter testing was done with a six digit Aligent meter.

Ok so the chart goes like this. Series resistance is to simulate the output of a guitar with different pickup. Go to Lollar and see where I was going with this. Yes if you use both neck and bridge the impedance will go down by X amount and change everything. The output voltage Vac rms is in the next column, the input voltage Vac rms is what is seen by the pedal (again the dac output is fixed at 0.119Vac). The -3dB roll off is determined by the output voltage * 0.707 and then the oscillator pushed down till that output voltage is reached. The gain is determined by the output/input and is numerical not in dB. You can do that calculation by 20*Log(out/in).

So in general the -3dB point goes lower with increased output impedance. In the case of the Pete Cornish model with a buffer -> TB that -3dB point would never change and is fixed I think at 2.5Khz from what I read.

Besides this my calculations for input impedance were not correct. Beano Boost is 18.04K and my Nitrous T is 17.832K

In the Medium output position the Beano -3dB is 670Hz and the Low is 400Hz. The Beano's caps are 6800pF root and 6800pF and 0.015uF. My Nitrous T mid point is 775Hz and my low is 335Hz. My caps are 5600pF root, 5600pF and 0.022uF. Of course the type and tolerances will vary somewhat and that will cause some differences.

Again my Nitrous T is an OC140 hfe 140, NPN design with a 10K transformer in the collector and a 10K volume on the secondary. From above I use the low drop out shunt regulator which is really nice as my self noise was down a ton. You can check out the schematic on the Beano in the other thread.

Thoughts?

Thanks,
Gordon

[Banjan73]

One comment:
In my reversed beano (schematics in the other tread) the caps are 5,6nF, 15nF and 6,8nF. And the transistor is CV7003 (OC44).
Another thing: I`ve just finished a EL84 high gain, two channel, effects loop amplifier build. As we discused in my tread, I thought the input impedance was closer to 1M, as I am used to that tube amp type of input (with the often used 1M pulldown resistor). But after reading your answers and the electro smash rangemaster analysis, I agree that the input impedance is much lower on the beano. Haven`t done the math, but I trust your engenering skills on this one.

[Wavelength]

Banjam,
I have the Beano Boost mini with Mullard transistors in it, one of the options. The caps in mine are 6800pF, 6800pF & 0.015uF. My Mullard transistor is a CV5713 in my beano. Some say that is an OC71 but I have a couple hundred of those and they are mostly an hfe of like 35-45. All the OC44's I have hfe between 100-150.

The electrosmash page is all theoretical and so the 12K input impedance and the 2.5Khz -3dB point was just math. Everything I did was through testing.

You would not want to put one of these in an effects loop! It has to sit right up front to see the impedance of the guitar. OR: you do a Pete Cornish thing with buffer input. I have been building amps for 40 years and I really don't get the idea of an effects loop. They all have different aspects and the signal level is always much higher than the expected signal level most pedal designers use.

I have about 50 Valvo and Mullard OC44's I may make an inverted design like the Beano and see how it sounds. I am pretty happy with the prototype and got a shipment of Amtrans carbon thin film resistors today and will finish up another one to send to Dan at TPS today. They are doing a special on these soon.

Thanks,
Gordon

[tonyharker]

The CV5713 is an OC72 equivalent and should look like this:


Other CV equivalents may be found here: http://www.wylie.org.uk/technology/semics/CVseries/CVseries.htm

[Banjan73]

Nonono! Off course I am not going to put my future beano copy in a loop!    This one is, if I use it at all, going in front.
Regarding a loop, my opinion is that it for certain pedals it is a must. I certainly wouldn't think of putting my DD-3 in front of the high gain stage, for instance.
I referred to an amp just to illustrate that 1M pulldown resistor. Nothing else.
I am currently fine tuning the pre shape filter of the high gain channel of my amp. Thats why i got to think about these treble boosters. They are somewhat working in a similar way (removing some bass frequencies)..
I did here the other day, test a mxr 6 band eq pedal in front of the amp. That also did the job.
The best way though is to tune the filter in the amp as I want i to sound. Then i dont need any of these pedals in front.

[teemuk]

The best way though is to tune the filter in the amp as I want i to sound. Then i dont need any of these pedals in front.

...And if you look at how e.g. classic Marshall amps evolved from mid 1960's to late 1970's you can see exactly this trend: More treble peaking filters, lower cathode bypass capacitances, etc. "Hot rod" mods are usually about adding a yet another treble boosting gain stage to input. It's necome a total trend to "bake in" that sort of EQ to nearly all amps with higher gain.

[Wavelength]

Teemuk,

I have sitting in front of me a new high gain 7591A amp combo (12" Alnico Warehouse). It has a switch on it just for that, a treble booster in the amp. Plus a switch to change the mid scoup frequency either up or down in the tone stack.

Thanks,
Gordon