Hello all. Mr. Price is probably knee deep with finals at MIT this week, so, I thought I'd post this question here, see if anyone with more knowledge than me can help (aka everyone).
Here is the problem. For both low-pass R/L filters, without a source input, I'm seeing DC offset start at 5.0v, and steadily climb past 10v, presumably, it will keep climbing.
I've attached the scematic and link for more information. Does anyone have any ideas? The HP L/R is steady at .03v for each channel. The regulated and unregulated power supply values check out as well. Thanks for any and all input.
Link to website
Here is the problem. For both low-pass R/L filters, without a source input, I'm seeing DC offset start at 5.0v, and steadily climb past 10v, presumably, it will keep climbing.
I've attached the scematic and link for more information. Does anyone have any ideas? The HP L/R is steady at .03v for each channel. The regulated and unregulated power supply values check out as well. Thanks for any and all input.
Link to website
An externally hosted image should be here but it was not working when we last tested it.
DC at input, or DC at gates of stage 1 and/or 2 of LP. There are series caps in HP to prevent this.
I'm guessing current sources are designed for 0 volts on output (if not then design error?), so perhaps there is an odd transistor that is pretty far off.
I'm guessing current sources are designed for 0 volts on output (if not then design error?), so perhaps there is an odd transistor that is pretty far off.
Shouldn't there be a method of DC bias on the input? I know fets don't need a bias as such, but with not being tied to anything at DC the gate will simply drift through leakage capacitance due to Cgate and C101.
tiroth said:
According to Mr. Price, with an input source, there is supposed to be some small AC on output.
If your source doesn't have a resistor to ground at the output, then I'd first tackle richie00boy's solution and see if that works.
The input scheme is quite bizarre IMO. As well as attenuating the input somewhat it also changes the frequency at which the lowpass rolloff occurs. As well as having no DC bias. Not exactly the example of best practice I'd expect from a MIT tutor!
Well, I'm a little confused then...
What some people are telling me here is, it very well may be a design flaw, and not something I screwed up, and they are suggesting adding to his design.
The original designer and supplier of this PCB, has been using this in his setup for a few months now. He's been helping me troubleshoot other problems, until recently when I assume he's become busy. I would think he would have discovered these design errors in his setup, and would have informed me of the changes by now.
The only changes I've been made aware of are adding output capacitors (.1uf) for the high-pass.
I would assume with a 10v DC offset that keeps climbing, I've done something terribly wrong, not a design flaw.
What some people are telling me here is, it very well may be a design flaw, and not something I screwed up, and they are suggesting adding to his design.
The original designer and supplier of this PCB, has been using this in his setup for a few months now. He's been helping me troubleshoot other problems, until recently when I assume he's become busy. I would think he would have discovered these design errors in his setup, and would have informed me of the changes by now.
The only changes I've been made aware of are adding output capacitors (.1uf) for the high-pass.
I would assume with a 10v DC offset that keeps climbing, I've done something terribly wrong, not a design flaw.
It's not a shining example of best practice IMO, but if connected to a source with DC coupled output then it will work fine. Maybe that's how it was intended to be used?
To troubleshoot if the floating input is a problem simply ground the input. If it stabilises then it's fine, if not then there's an error somewhere.
To troubleshoot if the floating input is a problem simply ground the input. If it stabilises then it's fine, if not then there's an error somewhere.
Mr. Price has contacted me and, yes, the offset should be fine when connected to a source. He's going to post here in a little while to clarify things.
I think I may try it out with a source tonite, if I get the time to do so.
I think I may try it out with a source tonite, if I get the time to do so.
Hi guys,
The input to this crossover does "float" at DC. However, it has never been a problem with any real world input source. I've tried all sorts of them including AC coupled ones - CD players, computers, Ipods, electric guitars. I am pretty sure it can be corrected with a 1M (or: any similarly large value) resistor from input to ground so the offset is always zero under any conditions. To be honest, I'd never even thought of it before. My mistake, if the input offset was worrying anyone.
The baffle step center frequency does shift with the level adjustment, but it's supposed to work that way! (The circuit has no insertion loss at low frequencies, only attenuating above 200Hz by ~2db or more.) It is just a simple "dirty" way to get more flexibility in tone control, since the tweeter level can also be adjusted, but for simplicity's sake there's no independent "bass" or "midrange" level control. I intended the crossover to have the most neutral sound in a typical room with the pot at half way (50k). It's a lot of fun to try variations here, given the clean sound of the system, many different settings of baffle step and driver levels can sound good.
I consider this crossover a piece of the "whole system" and I designed it to work with the Kit281 speakers just for fun. The circuit itself has worked wonderfully for me sound- and reliability-wise. I hope it does the trick - improving the midrange and giving the speaker a new more dynamic personality - for anyone who wants to try building it. If you're wondering, MIT has nothing to do with this. I just can't host a regular Comcast web space from inside their network. 🙂
Thanks
Michael P
The input to this crossover does "float" at DC. However, it has never been a problem with any real world input source. I've tried all sorts of them including AC coupled ones - CD players, computers, Ipods, electric guitars. I am pretty sure it can be corrected with a 1M (or: any similarly large value) resistor from input to ground so the offset is always zero under any conditions. To be honest, I'd never even thought of it before. My mistake, if the input offset was worrying anyone.
The baffle step center frequency does shift with the level adjustment, but it's supposed to work that way! (The circuit has no insertion loss at low frequencies, only attenuating above 200Hz by ~2db or more.) It is just a simple "dirty" way to get more flexibility in tone control, since the tweeter level can also be adjusted, but for simplicity's sake there's no independent "bass" or "midrange" level control. I intended the crossover to have the most neutral sound in a typical room with the pot at half way (50k). It's a lot of fun to try variations here, given the clean sound of the system, many different settings of baffle step and driver levels can sound good.
I consider this crossover a piece of the "whole system" and I designed it to work with the Kit281 speakers just for fun. The circuit itself has worked wonderfully for me sound- and reliability-wise. I hope it does the trick - improving the midrange and giving the speaker a new more dynamic personality - for anyone who wants to try building it. If you're wondering, MIT has nothing to do with this. I just can't host a regular Comcast web space from inside their network. 🙂
Thanks
Michael P
- Status
- Not open for further replies.