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Author Topic: Designing a Solid State Bass Amp (formerly "Transparency")  (Read 8260 times)

SECONDandBOWERY

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Designing a Solid State Bass Amp (formerly "Transparency")
« on: July 02, 2006, 08:10:32 PM »
What is the key to making a transparent preamp?

Let's say it's for bass.  The point is that no matter what bass you plug into it, it still sounds like that bass.  A P-Bass sounds like a P-Bass, a Jazz Bass sounds like a Jazz Bass, and Derek Smalls's doubleneck BC Rich mockingbird sounds like Derek Smalls's doubleneck BC Rich Mockingbird.  No fancy sound alteration, just the true natural sound of the bass but loud enough to pump it thru a poweramp.

I'm assuming that the fewer parts in the signal path, the more transparent the circuit will be, so a minimal amount of amplifying untils such as transistors and opamps as well as a minimal number of knobs should be used.  Am I right on this?  If not, please tell me what I'm wrong about/missing/forgetting/confused about.

Thanks!

Rock'n'Roll,
--Andy
« Last Edit: August 02, 2006, 10:01:20 PM by SECONDandBOWERY »

trevize

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Re: Transparency
« Reply #1 on: July 05, 2006, 04:23:19 AM »
if you look at high-end hi-fi power amps there's only a volume knob.  But we're speaking of power amp. A preamp HAS has a tone on it's own.

Concerning the small parts count i don't know. I suppose that for preamp simple is better but too simple probably it's not enough. Talking about bass you need a little bit of compression, clarity, and probably more than two gain stages.

Too much gain for one stage = distorsion
a lot of distorsion from only one stage = probably sounds ugly
a bit of distorsion from a lot of stages = probably more ear pleasing

That's just my 2 cent and i' no electronic expert.

SECONDandBOWERY

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Re: Transparency
« Reply #2 on: July 25, 2006, 03:03:10 PM »
If I used a high voltage to increase the headroom of the transistor/FET/OpAmp then couldn't I get away with using fewer parts and not having to deal with the distortion at all?

Rock'n'Roll,
--Andy

SECONDandBOWERY

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Re: Transparency
« Reply #3 on: July 27, 2006, 06:13:14 PM »
...Anyone?  I need some help with a transparent bass pre-amp here!

Rock'n'Roll,
--Andy

teemuk

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Re: Transparency
« Reply #4 on: July 28, 2006, 05:47:27 AM »
Consider this: How much gain do you need (to reach amplifier's input sensitivity) and what is the maximum peak voltage from the instrument. A basic opamp stage equipped with large decoupling caps, using highest supply voltage as possible (typically +-18V) and operating on low gain should be quite transparent. A good impedance matching (don't underestimate the importance of it) and filtering of the supply voltages is a must - as well as a grounding scheme that will not hum. You need this and a volume potentiometer. Now you have a transparent preamp.

Note: Bass guitars can put out large voltage peaks, i.e. 2 volts. With single opamp circuit this limits the gain to theoretical 9. (2 x 9 = 18 V) - A gain of 8 is more realistic figure since opamps never swing full rail. This figure is actually quite high considering the fact that power amps usually have an input sensitivity lower than 1 V, so you have no need for massive gain. Typically, most of the preamp gain is just used to compensate the losses from passive circuits (tone stacks etc.)

SECONDandBOWERY

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Re: Transparency
« Reply #5 on: July 28, 2006, 07:25:23 PM »
Decoupling caps are essentially the output cap, right ('cause coupling caps surround the gain stage and "de" implys the end, right)?  How large is a  large decoupling cap?  What value cap provides a flat frequency response for a bass? 

What's the easiest way to match impedances?

Do you think that two opamp gain stages with a Baxandall tone stack in the middle would still be fairly transparent?

See the attached schematic.  I think I did it right...

Rock'n'Roll,
--Andy
« Last Edit: July 29, 2006, 02:02:18 AM by SECONDandBOWERY »

teemuk

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Re: Transparency (In Relation to Bass Preamps)
« Reply #6 on: July 29, 2006, 05:04:33 AM »
The decoupling capacitor value depends on RC time constant - which is basic circuit theory. Small load impedances require higher decoupling capacitance (i.e. 4 ohms load requires thousands of microfarads for decoupling) and higher load impedances - as in preamp circuitry - require less. In general, a higher decoupling capacitance extends the low end frequencies.

About impedance matching: You should know your pickup's impedance as well as the input impedance for power amp. Preamp's input impedance should be higher than pickup's impedance (otherwise you will attenuate certain frequencies). Same thing with preamp - the power amp input impedance has to be higher than preamp's output impedance. However, best power transform happens when both output and input impedances are equal.

Generally, tonestack circuits are not very transparent (although Baxandall TS can be tweaked to offer quite linear output when the potentiometers are in middle position). They also introduce quite huge signal losses unless they are done with active circuitry.

The schematic: Try something like 4.7uF - 10uF insteaf of 470nF. Your circuit attenuates bass frequencies very heavily. (Almost zero gain for 20 Hz). Any low noise, fet-input opamp would work well.

trevize

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Re: Transparency (In Relation to Bass Preamps)
« Reply #7 on: July 29, 2006, 05:37:54 AM »
the opamp could be a ne5534, or even a tl071

SECONDandBOWERY

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Re: Transparency (In Relation to Bass Preamps)
« Reply #8 on: July 29, 2006, 07:11:06 PM »
In that case, the 470nF will be changed to a 10uF.  Using Duncan's Tone Stack Calculator, I did in fact find a way to get the Baxandall's output really close to being linear, so here's my idea:

Input -> Gain Stage 1 -> Baxandall Controlls -> Volume Controll -> Gain Stage 2 -> Output

Gain Stages 1 and 2 would be identical and all coupling caps would be around 10uF.  I'd probably end up using a TL072.  I'd like to run it into a phase inverter and then a bridged power amp (using a LM4780... or two).

Does that look like it'd work out?

Rock'n'Roll,
--Andy
« Last Edit: August 02, 2006, 06:40:30 PM by SECONDandBOWERY »

SECONDandBOWERY

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Re: Transparency (In Relation to Bass Preamps)
« Reply #9 on: August 02, 2006, 06:41:17 PM »
Here's a schematic for the idea so far...

Rock'n'Roll,
--Andy

joecool85

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Re: Transparency (In Relation to Bass Preamps)
« Reply #10 on: August 02, 2006, 07:54:50 PM »
How'd you make the schematic?  It looks great.
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SECONDandBOWERY

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Re: Transparency (In Relation to Bass Preamps)
« Reply #11 on: August 02, 2006, 09:49:01 PM »
How'd you make the schematic?  It looks great.

http://www.expresspcb.com/ has a bundle available for a free download.  ExpressPCB is the PCB maker, and ExpressSCH is the schematic creator.  It took me a while to get it so all the components looked the way I wanted them too, but using the ones from the library that comes with the software works just as good.

But anyway, does that circuit look like it would function decently?  I'm out of town at the moment, but i'll be home next week and would like to start breadboarding.

Rock'n'Roll,
--Andy

teemuk

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Re: Designing a Solid State Bass Amp (formerly "Transparency")
« Reply #12 on: August 03, 2006, 03:27:48 AM »
There's something fishy in those buffer opamps. I assume they are dual so in the positive leg of the non-inverting buffer you probably would have to clamp the positive input to ground. The negative inverter will not work that way; as it is now it just bypasses the opamp. You need an inverting opamp circuit with a unity gain.