Please help me understanding this Ashdown passive tone stack

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Please help me understanding this Ashdown passive tone stack

Postby wgen » 12 Jan 2017, 13:45

Hello my dear,
new user here! I'm opening this thread because I'm desperately searching for some help.
I have the schematic of the little Ashdown bass combo, the Perfect 10 (the most recent one they made, just a couple of years ago)...and I found the tone stack part rather unusual. I'm posting this thread in this section because I thought that it could be of some interest for some DIY design experiments.
Let me say, first of all, that I'm just a player with great curiosity about amplifiers and effects pedals...but I'm a noob in understanding circuits.
Could you please help me understanding how to analyse the frequency response of this tone stack? This one seems quite different from classic Fender or Marshall tone stack which I usually check with the Duncan tone stack calculator...
This is the part of the schematic with the tone stack I talked about:
AshdownToneStack.png


Thank you very, very much in advance to anybody who will suggest me anything about this!
Please, excuse me for my English, I hope that this post was clear enough.
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Re: Please help me understanding this Ashdown passive tone s

Postby phatt » 13 Jan 2017, 12:36

Hi Wgen,
It's an *Active* tone circuit.
Changing the values of R26, C16, C17 and R27 will alter how the tone controls respond.
It's a fairly common circuit design so nothing new for those who design circuits.
Yes these are not like the old classic Fender and Marshall circuits which are passive and very lossy. You can only Cut with those but they tend to be better for guitar.
Active means no loss which can be useful,, but not always.
Some players actually use both passive and active tone shaping to get the best of both circuits.
BTW, Your English is fine :thumbsup
Phil.

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wgen (13 Jan 2017, 14:56)
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Re: Please help me understanding this Ashdown passive tone s

Postby wgen » 13 Jan 2017, 15:07

Hello Phil, thank you very much for the reply!
Yes, even though I'm really not a tech by any means, I had thought that, with the negative feedback of an opamp involved, it isn't passive.
I wrote passive because I found it called "passive tone stack" everywhere on the Ashdown webpage and in the manual, so it seems that sometimes there isn't much useful information from the manufacturers :lol:
By the way, do you eventually know how I could make up my mind about how these potentiometers shape the tone...?
Is there some calculator online I could use for this application?
I searched around on the web, there are some calculators, apart from the Duncan tone stack calculator, I found something on the Ampbooks webpage, and also on the Muzique Lab Notebook section, there are some gyrator and rc filters calculators...but I really couldn't tell if I might find some ideas about the tone shaping of this part of the schematic, from those.
Thank you very much anyway!
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Re: Please help me understanding this Ashdown passive tone s

Postby alexradium » 13 Jan 2017, 15:38

the fastest way to see how this circuit behaves is to simulate it with a spice software.
Anyway in bass circuits you have bass and treble usually at 40/50Hz and 5/10KHz,then middle frequencies can be anything between 400 and 1K

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wgen (13 Jan 2017, 16:18)
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Re: Please help me understanding this Ashdown passive tone s

Postby wgen » 14 Jan 2017, 00:41

alexradium wrote:the fastest way to see how this circuit behaves is to simulate it with a spice software.
Anyway in bass circuits you have bass and treble usually at 40/50Hz and 5/10KHz,then middle frequencies can be anything between 400 and 1K


Thanks for the reply!
I downloaded LtSpice online for free, and I started today with the circuit drawing of this little part of the schematic...It's my first time with these kind of analysis.
If you know something about this type of analysis...where should I click on to achieve an overall frequency response plot from this tone stack?
If I click on the I(OUT) point of the opamp, the one just under the R28 resistor in the schematic, the plot shows me an almost flat response (with a little difference of about 1 dB only).
If I click on different settings for the 50kohm potentiometers (for example, being all of those 50k ohm pots, I put in the circuit drawing two 25k fixed resistors, trying to simulate a linear 50k ohm potentiometer at the center position), then, instead, the plot shows some tonal shaping. So that, if I click on the two 25kohm resistors, which simulate the 50kohm pot at the center position, for the treble control for example, the plot shows some boost or cut of treble.

What I'm trying to say is, shouldn't the I(OUT) point of the opamp be the point which tells effectively the overall frequency response from the tone stack, instead? When I tried, in fact, the plot didn't change from that almost perfectly flat response I said above, even when I re-drew the circuit with the potentiometers either fully closed or opened.
I thought that the I(OUT) node of the opamp is the point from where the signal comes out from the tone stack section and then flows to the next section in the schematic.
What's wrong with it? I'm doing something wrong for sure, but I really don't understand why, or where.
I hope that my doubt is clear enough..!
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Re: Please help me understanding this Ashdown passive tone s

Postby alexradium » 14 Jan 2017, 15:50

well,the simpler way to capture the frequency response is this:
use a software like Visual Analyzer and send a sine wave from your pc line out to the amp,from the DI of the amp go to the line IN of the audio card and watch with the oscilloscope the resulting gain for every frequency or better you can sweep through the whole audio band to catch the EQ curve.
Otherwise you can use a smartphone,either to generate sine wave,with an app like Signal Generator, and Frequensee to evaluate the sound directly from the speaker.

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