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Setting up a ESP active crossover with gainclone
Setting up a ESP active crossover with gainclone
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Old 15th August 2007, 08:34 AM   #31
Dan_Steele is offline Dan_Steele  United Kingdom
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They are Peter Daniel LM3875 Premium GC's.

By low pass I take it we are refering to a Zobel?

Well I tried inserting a 1uf and 1ohm resistor (peter reccomends 2.7uf but I didnt have any to hand)

The problem was still there after this. If i made a difference it was only marginal.

No DC blocking as far as im aware, currently wired as per PD's instructions. - However the 1uf caps on the output of the crossover should take care of this?

I tried a CD with 0 to 20Khz sweep on it and it either made the amp unstable (sounded like clipping (or the CD was skipping - I should test it in another player) but more of a pulses white noise essence to it) - However this has only happened once during general music playback
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Old 15th August 2007, 09:15 AM   #32
pinkmouse is offline pinkmouse  Europe
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Setting up a ESP active crossover with gainclone
That's the output Zobel, that shouldn't affect things that much. PD's circuit is very nice, but it does lack a few things that can help stability, as I found out when experimenting with GCs in my old house, just a mile away from the Crystal Palace TV transmitters.

Okay, there's a couple of things we can try, first thing is to dowload the datasheet. If you look at the typical applications schematic, you will see a 220p cap across the inputs. You can add that very easily just by tacking it onto the bottom of the board and it reduces the amp's sensitivity to RF, not so much a problem in the wilds of Canada, but it can be an issue in the EMI crowded UK.

The other thing to try is the resistor and cap to ground shown at the tail of the feedback network. This reduces the DC in the feedback and makes the amp more tolerant to different preamps.

I'm not saying that adding these will solve your problem, fault finding at a distance isn't straightforward at the best of times, I much prefer to have the circuit in front of me with the scope and signal generator fired up and ready to go.
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Old 15th August 2007, 12:14 PM   #33
BWRX is offline BWRX  United States
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Quote:
Originally posted by Dan_Steele
Im using a 50k alps log for volume and 20K trimpot as sugested.
...
At the current levels (trimpot positions) as the volume is increased it sounds like one is tuning a radio in (that varying tone). When 12 oclock is reached all his disspears and the whloe system sounds perfect, when around 3 oclock is reached the hiss comes back for the last 1/4 turn. (however all hum in the mid/low amp diminishes)
For clarification, the trimpots are the ones in the buffer stage as shown in Figure 3 here http://www.sound.westhost.com/project09.htm, correct? And your volume pot connected between the output of the active crossover and the input of the amp, right?

If so, your 50k volume pot is not a good match for the 20k input impedance of the noninverting LM3875 circuit. It would be better if you used a lower value pot, 10k or lower if possible. You may also try grounding the case of the alps pot (if you haven't already) and wiring in a small cap across the inputs of the LM3875 as pinkmouse suggested.
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Old 15th August 2007, 12:19 PM   #34
Dan_Steele is offline Dan_Steele  United Kingdom
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Hi BWRX,

The trimpots are located on the board and connected before the output buffer as shown.

The 50K pot is not after the Xover, but before, therefore only a dual pot was required (would need a quad if on the output side)

The pot comes after a elma selector switch which has every other input shorted as suggested by morgan jones.

This lends itself to miniumum wiring lenghts in the preamp without the use of shaft externders.

I temporarily have a earth on the alps case, this prevents hum when the volume knob is touched

Im going to have a go at pinkmouses suggestions, just ordering the 220pf's now.

Regards

Dan

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Old 15th August 2007, 12:23 PM   #35
BWRX is offline BWRX  United States
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Hi Dan.

Yes, that makes more sense that the volume pot is at the input. I wasn't thinking that you'd need a 4 channel pot if it were on the output of crossover...

Well, don't worry about it's value then But do try grounding the metal part of the pot and adding the cap across the inputs of the LM3875.
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Old 15th August 2007, 02:01 PM   #36
pinkmouse is offline pinkmouse  Europe
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Setting up a ESP active crossover with gainclone
Dan, if I'm not too late, I'd recommend silver mica or polystyrene type caps.
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Old 15th August 2007, 02:52 PM   #37
Dan_Steele is offline Dan_Steele  United Kingdom
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I've ordered some polyprop's and also polycarbonate types.

Theres some 220pf silver micas on ebay at the moment but they looked a little 'vintage'

I steered clear of ceramic as I've can remember reading it has no place in an audio signal path (which I guess this application is along those lines)

Dan
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Old 15th August 2007, 04:56 PM   #38
BWRX is offline BWRX  United States
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The quality of ceramic caps depends on the dielectric. C0G/NP0 types are fine but come in small values. X7R and lower are best used for decoupling.
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Old 17th August 2007, 08:41 AM   #39
Dan_Steele is offline Dan_Steele  United Kingdom
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Right Guys, an Update;

The 220pf polyprops arrived y/day I installed one to one of the amps and it changed all of them for the better (disconecting the one with the cap brought the hiss back)

So I then went on to install them in all four amps, I am pleased to say that the radio tuning / bizzare crackling has gone (all but on the last 10th of a turn on the pot, but im satisfied at the moment as it will never go that high)

There was still a low frequency buzz that was annoying me, after searching high and low, I decided to see what the amps are going to look like when in there finished position (ontop of the power supplies) and low and behold the vast majority of the buzz dissapeared!

I also tried the resistor and cap in the feedback loop to eliminate DC, but this made no audible difference.

Now there is still a hiss in the background, but only around 10% of what was there before, but I'm confident im moving in the right direction to compeltly get rid of it.

Here are a few pics I took last night (they were taken on my mobile mind)
Heres a pic of two of the amps

Click the image to open in full size.

And one of the X-over and PSU

Click the image to open in full size.

This is the whole setup, showing the amps on top of the psu's
Click the image to open in full size.
Every PSU has a safety earth connected, however there is no connection between the amps and their chassis at present.
The cables between the amps and the PSU's are screened and connected to the safety earth.
The PSU's still need the rear feet attaching and so does the x-over. Also, I will make the earth connection shown with the croc clip permanent and change over two of the outputs. And trim down the pot / selector shafts to get a better fit with the knobs.
Finally, I've bought some 8mm screened cable for interconnects and will be making my own of the shortest length possible to hopefully further reduce the noise.


Finally

Click the image to open in full size.

These will be finsihed soon and hopefully I will be able to sleep / see my girlfriend then!

All in all, not bad for a 19 year old if I dont say so myself


Thanks guys you've really helped me here!

Edit: Also had some screened IEC cables arrive yday so they may have added to the overall effect
Edit2: There is a chip shaped hole in the base of each amp so that the back of the chip is directly coupled to the heatsink with a dab of compound in between
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Old 17th August 2007, 01:57 PM   #40
pinkmouse is offline pinkmouse  Europe
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Setting up a ESP active crossover with gainclone
Nice job, well done.
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