Hi guys, long time lurker here. Am trying to modify an old audison amp (SRx-4 - the early model with the silver-and-blue color scheme) which has a rather narrow range of crossover frequencies, namely 50-220 Hz for LP and HP, offering the choice of HP for Channels A L/R and HP/LP for ChB L/R. I'd like to use it to actively drive my mid-woofers and tweeters separately using the active crossover, as I have another amp for the subwoofer. Which means, modifying that active crossover frequency - the later model of that amp has a "10x" switch which does change the filter range, something like that is what I want to do (although permanently).
Since I'm not that experienced with amp modification, I'd like to ask here first before making potentially money- or time-consuming mistakes. As I understand it (haven't been able to trace the dual-layer SMD PCB yet), the circuit resembles what is shown in Elliott Sound Products Project 148 - some variable resistors combined with a capacitor to set the filter frequency. So my approach would be to solder in new caps of much smaller size, basically going ~1/25th C to give me 25x the crossover frequencies (1200-5500Hz). I'm 99% sure the 56nF 100V caps here are the ones in the HP filter section, the sliders on the bottom of the image are the ones controlling the frequency. However since there are clearly lines going from the sliders to the set of 4 caps (82nF 63V types) on the bottom right of the image, I could easily imagine the 56nF being used in the high-pass sections, and the 82nF ones for the lowpass section (which doesn't quite explain why there are different values on these than on the others).
Does anyone have the schematics available? It's a bit of a daunting task to retrace this PCB for an amateur like me.
Does anyone foresee any problems with replacing them with either 2.2nF film caps (~1/25th) for the 56nF ones, and 3.3nF for the 82nF caps?
Also, to protect the tweeters when driving them actively, would you guys recommend installing a large cap (basically a 6dB HP filter active below the working range, like 100uF~400Hz xo frequency), or does that seem like overly protective and not strictly necessary on a better-quality amp like the audison?
So thanks for reading through, looking forward to any replies!
Since I'm not that experienced with amp modification, I'd like to ask here first before making potentially money- or time-consuming mistakes. As I understand it (haven't been able to trace the dual-layer SMD PCB yet), the circuit resembles what is shown in Elliott Sound Products Project 148 - some variable resistors combined with a capacitor to set the filter frequency. So my approach would be to solder in new caps of much smaller size, basically going ~1/25th C to give me 25x the crossover frequencies (1200-5500Hz). I'm 99% sure the 56nF 100V caps here are the ones in the HP filter section, the sliders on the bottom of the image are the ones controlling the frequency. However since there are clearly lines going from the sliders to the set of 4 caps (82nF 63V types) on the bottom right of the image, I could easily imagine the 56nF being used in the high-pass sections, and the 82nF ones for the lowpass section (which doesn't quite explain why there are different values on these than on the others).
Does anyone have the schematics available? It's a bit of a daunting task to retrace this PCB for an amateur like me.
Does anyone foresee any problems with replacing them with either 2.2nF film caps (~1/25th) for the 56nF ones, and 3.3nF for the 82nF caps?
Also, to protect the tweeters when driving them actively, would you guys recommend installing a large cap (basically a 6dB HP filter active below the working range, like 100uF~400Hz xo frequency), or does that seem like overly protective and not strictly necessary on a better-quality amp like the audison?
So thanks for reading through, looking forward to any replies!
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The amp likely has DC offset protection but I would recommend using a series capacitor to protect tweeters.
Is this the sallen key circuit or the state variable circuit circuit (compare to attached)?
In the future, please break up the paragraphs. All text in one block makes it difficult (for me) to read).
Is this the sallen key circuit or the state variable circuit circuit (compare to attached)?
In the future, please break up the paragraphs. All text in one block makes it difficult (for me) to read).
Attachments
Ah thank you, already broke up the paragraphs a bit, perhaps it also helps others to read through that post.
I'd say it mostly resembles the state variable variant, as there is no way to set Q or filter steepness on these. I know Audison added more complicated filters for the SRX1-3 channel variants, to allow for better incorporation of subwoofers, but the 4ch variant I have only offers 12dB variable frequency. And knowing that, I came to the unsurprising conclusion of being bit of a dummy here - a 12db Filter will always use 2 caps per channel, so to filter 4 channels with 12dB, one definitely needs 8 caps - so I strongly suppose the 82nF bank is also involved.
I'd say it mostly resembles the state variable variant, as there is no way to set Q or filter steepness on these. I know Audison added more complicated filters for the SRX1-3 channel variants, to allow for better incorporation of subwoofers, but the 4ch variant I have only offers 12dB variable frequency. And knowing that, I came to the unsurprising conclusion of being bit of a dummy here - a 12db Filter will always use 2 caps per channel, so to filter 4 channels with 12dB, one definitely needs 8 caps - so I strongly suppose the 82nF bank is also involved.
Hmm interesting - correct me if I misunderstand this, but from my practical measure-ability, assuming the sallen key filter, the caps within a filter block (like C75/C76 on the sallenkey schematics) should show a direct connection; for the state variable filter like C34 and C40 on the state variable schematics, there is definitely a kohm resistance between them.
On my board, I find a direct connection between pairs of 82nF caps and a noticable resistance between all the 56nF caps - so I guess they used both architectures there, the 82nF being used in the channels 1+2 HP as sallen key, and the 56nF use state variable for the switchable LP/HP for Ch 3+4... At any rate, the replacements capacities should be the same (1/25 the original capacity to generate 25x the original cutoff frequency), right?
On my board, I find a direct connection between pairs of 82nF caps and a noticable resistance between all the 56nF caps - so I guess they used both architectures there, the 82nF being used in the channels 1+2 HP as sallen key, and the 56nF use state variable for the switchable LP/HP for Ch 3+4... At any rate, the replacements capacities should be the same (1/25 the original capacity to generate 25x the original cutoff frequency), right?
There are likely filter calculators online. The analog line level forum on this site may be of some help, after you determine what you have.
https://www.diyaudio.com/community/forums/analog-line-level.128/
Someone here may have the exact diagram. Give them a day or so.
25x may not be a viable scale. You may only be able to go 5x or 10x without having to change resistor and maybe pot values.
Would an external crossover be a viable option, for you?
https://www.diyaudio.com/community/forums/analog-line-level.128/
Someone here may have the exact diagram. Give them a day or so.
25x may not be a viable scale. You may only be able to go 5x or 10x without having to change resistor and maybe pot values.
Would an external crossover be a viable option, for you?
Yes, if anyone had the actual precise schematics for that amp available that would of course help - but I already have to thank you for the SRX2-ones, I think they cleared it up for me fairly well, especially since even the cap values match what I have on my board, and on further test I found a direct connection between opamp inverting input (pin 2), a 56nF cap and the variable resistor -> state variable, while the 82nF caps connect to non-inverting opamp input pin3 -> sallen key.
I found a calculator for Sallen-Key-Highpass-Filters [link], which seems to generate exactly the frequency change of f ~1/C assuming the resistors are kept constant, and for the state variable (which is the same as the already mentioned ESP Project 148 [link]) I had that relationship of f ~ 1/(R*C) in mind all along.
I agree that a 25x change could be a bit much, and only take that into consideration because it would make the crossover range very flexible for tweeter selection. I would probably be fine with about 12x, which should give an upper cutoff-limit of 2.6kHz - which is probably fine for the two or three tweeter candidates I have in mind.
On a sidenote: I'd rather have the tweeters coupled low anyways; coming from more of a HIFI background, I always consider car audio situations to combine extreme nearfield with rather disparate speaker placement and odd off-axis listening positions, which of course makes it very hard to achieve a point source characteristics with even moderate crossover frequencies - so I always wonder why car tweeters routinely get crossed over at like 3.5 - 5kHz, way higher than most 2-way hifi speakers??? maybe I'm missing something important there, is any combing/that somehow masked out in that upper range by the other oddities of the car geometry?? Probably have a bit of experimentation to do.
So anyways, this 12x change should be achievable using 56nF-> 4.7nF, and 82nF->6.8nF replacements, which looks a bit more palatable. I just ordered some of those, worst case I'm now out €10.
An external crossover would of course work, the amp certainly has the inputs for it... I just figured, why install a new box instead of simply modifying what I have and possibly learn something while doing so.
I found a calculator for Sallen-Key-Highpass-Filters [link], which seems to generate exactly the frequency change of f ~1/C assuming the resistors are kept constant, and for the state variable (which is the same as the already mentioned ESP Project 148 [link]) I had that relationship of f ~ 1/(R*C) in mind all along.
I agree that a 25x change could be a bit much, and only take that into consideration because it would make the crossover range very flexible for tweeter selection. I would probably be fine with about 12x, which should give an upper cutoff-limit of 2.6kHz - which is probably fine for the two or three tweeter candidates I have in mind.
On a sidenote: I'd rather have the tweeters coupled low anyways; coming from more of a HIFI background, I always consider car audio situations to combine extreme nearfield with rather disparate speaker placement and odd off-axis listening positions, which of course makes it very hard to achieve a point source characteristics with even moderate crossover frequencies - so I always wonder why car tweeters routinely get crossed over at like 3.5 - 5kHz, way higher than most 2-way hifi speakers??? maybe I'm missing something important there, is any combing/that somehow masked out in that upper range by the other oddities of the car geometry?? Probably have a bit of experimentation to do.
So anyways, this 12x change should be achievable using 56nF-> 4.7nF, and 82nF->6.8nF replacements, which looks a bit more palatable. I just ordered some of those, worst case I'm now out €10.
An external crossover would of course work, the amp certainly has the inputs for it... I just figured, why install a new box instead of simply modifying what I have and possibly learn something while doing so.
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Ok, parts arrived and got installed yesterday, it definitely changed the filter frequency up high(er) and I now only have to test for the actual range and left-to-right evenness - the caps are only available as +/-5% types, so I did the old "order 10 caps instead of 4 and arrange them by measured capacity, then select the ones in the middle" to get closely matched, but only testing will tell if it worked as intended.
Thank you again Perry, for your very useful input!
Thank you again Perry, for your very useful input!