JLM 60Watt Power AMP Kit

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chrisp
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Post by chrisp »

Joe Malone wrote:When you get the speakers finished try removing the 1.5mH inductor and 4ohm resistor and wiring direct to the driver from the AMP and adding a 180nF cap in series with 2.2k resistor across the +vol- point as this will give the same step in the frequency response.

Let me know what you find as both ways will sound different due to the uneven load of the speaker to the inductor and the added resistance in series with the driver will also affect the damping.
At long last I had the chance to open up my monitors and try this out. Could I just check that the cap and resistor are in series? 'cause the inductor and resistor were in parallel between the amp out and the speaker+. The cap (0.18uf MKT) / resistor (1% 2k2) in series across +vol- deadened the sound quite significantly. I also auditioned the units with no compensation circuit - they were more comparable to the inductor comp circuit but with a definite sense of more treble than bass. The best sound so far comes from the inductor/resistor circuit. Any thoughts?
Chris P
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Joe Malone
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Post by Joe Malone »

HI Chris
Yes the parts go in series and will take away some gain as it is leaving the low end and cutting back the highs to do the same as the passive inductor circuit.

Normal gain can be restored by changing the 1k resistor to smaller value.
Joe :-)
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chrisp
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Post by chrisp »

Hmmm

Compared to the reference (with inductor/resistor filter post amp)

At 2k2, the highs were distinctly attenuated;

At 1k, even more so;

At 0k (ie just the cap), barely audible.

At infinite k (ie open circuit across the pins), the treble was accentuated.

So I figured that a higher R than 2k2 was required. I wired a 100K pot in parallel to try this out.

There is indeed a close match to the volume and tone with R at around 73K!! Although the value is far from exact due to the lack of pot sensitivity.

Any suggestions as to what is going on here? I was not expecting this as a result.
Chris P
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Re: JLM 60Watt Power AMP Kit Now Available

Post by Joe Malone »

HI Chris
I think I did those figures with the 2 input resistors being 1k where as they are 10k on the normal amp PCB which would make the Resistor 22k instead of 2k2 and the CAP would be 18nF instead of 180nF. I don't have the info with me but from what you said above this sounds like it would be the problem. So try 22k in series with 18nF. The amp sensitivity will be 6dB less but that should do the same bass boost as the passive LCR circuit.
Joe :-)
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chrisp
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Re: JLM 60Watt Power AMP Kit Now Available

Post by chrisp »

Joe

Them's the numbers! Made up the filter as suggested, and yep, it's the same sound as the original.

I'm back in love with my monitors.

Thanks, I feel like I owe you something for all your number crunching.

Regards

Chris
Chris P
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Re: JLM 60Watt Power AMP Kit Now Available

Post by Joe Malone »

Cool :D
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chrisp
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Re: JLM 60Watt Power AMP Kit Now Available

Post by chrisp »

I just rebuilt my guitar amp using a Celestion 10" vintage driver and this amp kit.

The build was going exactly as per Joe's diagram, except with an added JLM1:4 transformer and XLR socket to provide a balanced DI out function from the input (whether or not that input is balanced TRS or, as in this application more likely, unbalanced TS).

Only problem was that when I switched it on, it was quiet at low volumes but quickly grew a major hum problem as the volume was turned up. The problem was in the mounting position of the volume pot. I realised that it was sitting directly above the toroidal tranformer, where the wires exit - the worst place it could have been to pick up power hum!

Now I could have simply moved the pot to another, less noisy position, but functionally, this layout was nice. Instead I swapped to some two core shielded microphone cable. The two inner cores go as per the diagram, and the shield at the PCB goes to the 0V conveniently next door. At the pot end, the shield does NOT go to the third leg of the pot - it goes to the pot body.

Noise problem gone, amp works a treat with the Celestion driver, and after many years I now have a combo I can say sounds good. Thanks again to JLM for a great kit.
Chris P
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Re: JLM 60Watt Power AMP Kit Now Available

Post by Joe Malone »

:D 8)
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greenmanhumming
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Re: JLM 60Watt Power AMP Kit Now Available

Post by greenmanhumming »

I'm thinking about maybe building 6 amp kits to triamp some speakers I have.... (old linn kabers) that i am using as a second set of studio monitors

this is a much more critical application than the mini PA I used them for before so I'd appreciate any tips on optimizing sound quality.

what are the pros and cons of more or less filtering caps?

related question - what are the pros and cons of separate transformers for each pair or even each AMP?

I have my eye on a 225va toroidal that i thought I'd use one of for each pair, but if a different configuration or more power will give better sound i'm open to suggestion.

I won't be running the amps hard, but I do want optimum sound.

Any tips on applying a crossover filter before each amp so I can bypass the passive crossovers in the speakers would also be appreciated...

one big heatsink or several separate ones? any difference?

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Joe Malone
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Re: JLM 60Watt Power AMP Kit Now Available

Post by Joe Malone »

greenmanhumming wrote:I'm thinking about maybe building 6 amp kits to triamp some speakers I have.... (old linn kabers) that i am using as a second set of studio monitors

this is a much more critical application than the mini PA I used them for before so I'd appreciate any tips on optimizing sound quality.

what are the pros and cons of more or less filtering caps?
It depends on how many run on one transformer. The more AMP PCB the better the uF total. As each AMP PCB has 4000uF per rail. We do have a special cap PCB that holds 4 x 10000uF 50v cap (20000uF per rail) and a 10Amp bridge rectifier on it for smoothing and then run that to the amp modules. But if you ran all 6 AMPs on one transformer you would have 20000uF total already.
related question - what are the pros and cons of separate transformers for each pair or even each AMP?
Having separate left and right or xover band transformers will definitely be able to show in testing as deeper channel/band separation but it is so low that listening wise it would be undetectable.
I have my eye on a 225va toroidal that i thought I'd use one of for each pair, but if a different configuration or more power will give better sound i'm open to suggestion.
If going to split the design at all I would use a 225va per left and right not per band. The power amp IC has great power rail rejection that all 6 amps could run on one 300VA or 500VA transformer just fine with great seperation.
I won't be running the amps hard, but I do want optimum sound.
Yes to optimize always take the power feeds to each amp from a star point at the bridge or output of bridge/cap filter PCB and star ground point and take your -neg speaker feeds from the star ground point as well.
Any tips on applying a crossover filter before each amp so I can bypass the passive crossovers in the speakers would also be appreciated...
Would need to see the passive crossover circuit and have resistances of inductors inline with the woofer to workout what be the best way to go.
one big heatsink or several separate ones? any difference?
One big heatsink is fine and use the biggest within reason you can to keep the operating temp down and away from the amps thermal protection circuit threshold.
Joe :-)
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greenmanhumming
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Re: JLM 60Watt Power AMP Kit Now Available

Post by greenmanhumming »

thanks Joe for the info.

seems like you are saying more power supply capacitance is always better.... some people advocate very small capacitance, as low as 1000uF for chip amps... so I wondered if theres a tradeoff somewhere.

why would you split left and right rather than per band? seems to me intermodulation between bands is likely a more offensive sound than between channels... but it may be so low as to be academic...

you say I can run all 6 off 300VA, thats only 50VA per AMP - but is there an optimum VA per channel? what is the point at which overkill ceases to provide any advantage?

I can't find the crossover schem anywhere, when i get time i will pull it out and draw the schem, its a big relatively complex beast... presumably with steep slopes, and 3 bands, makes the speakers hard work to drive passively.

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Re: JLM 60Watt Power AMP Kit Now Available

Post by Joe Malone »

greenmanhumming wrote:thanks Joe for the info.

seems like you are saying more power supply capacitance is always better.... some people advocate very small capacitance, as low as 1000uF for chip amps... so I wondered if theres a tradeoff somewhere.
4000uF per rail is the minimum I found to give ok ripple and less power rail sag for full power when using a 80VA to 100VA per amp.
why would you split left and right rather than per band? seems to me intermodulation between bands is likely a more offensive sound than between channels... but it may be so low as to be academic...
So I could build mono blocks that sit behind each speaker or are built into each speaker so cable length to the speakers are a short as possible. Remember intermodulation into each driver from the front panel vibration will be a 1000 times greater than that due to sharing a power rail. Best way to stop crosstalk between amp modules with one transformer is fully regulated power rails or huge cap banks to keep the power supply impedance super low. And use star ground and power feeds.
you say I can run all 6 off 300VA, thats only 50VA per AMP - but is there an optimum VA per channel? what is the point at which overkill ceases to provide any advantage?
Depending on the crossover of course but in general if the woofer amp get to full power the mid amp will be at 1/2 power or less and the tweeter amp will be at 1/4 power or less.
I can't find the crossover schem anywhere, when i get time i will pull it out and draw the schem, its a big relatively complex beast... presumably with steep slopes, and 3 bands, makes the speakers hard work to drive passively.
Yes to try to emulate steep crossovers is harder but eq in the crossover can be tricky and also trying to keep the dampening to the woofer the same so it tuning still works in the same box if difficult without actual testing.
Joe :-)
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greenmanhumming
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Re: JLM 60Watt Power AMP Kit Now Available

Post by greenmanhumming »

thanks again for the reply

Its a 2 and a half way system... by that I mean that there are 2 identical woofers and a tweeter, but the woofers have different crossovers, one handles just bass, and the other bass and mids.

so the mid/bass driver crossover does not have a hipass filter. and the two bass driver's amps will be working similarly hard.

I was thinking of building some of these for crossovers.. http://sound.westhost.com/project09.htm do you think its suitable?

the box is a sealed design, small and heavily damped, little tuning involved I expect..

after reading your answers I am tempted to just build it all running off a single 500va transformer.

if i were to try more caps how should they be added, some physically close to each channel? or does it not matter assuming wiring is good?

the speakers are only about 2m apart anyway with the amp in between... probably will always be something similar... so speaker cabling isn't a big issue i wouldn't think...

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Re: JLM 60Watt Power AMP Kit Now Available

Post by Joe Malone »

greenmanhumming wrote:thanks again for the reply

Its a 2 and a half way system... by that I mean that there are 2 identical woofers and a tweeter, but the woofers have different crossovers, one handles just bass, and the other bass and mids.
Yes this is fairly common.
so the mid/bass driver crossover does not have a hipass filter. and the two bass driver's amps will be working similarly hard.
ok
I was thinking of building some of these for crossovers.. http://sound.westhost.com/project09.htm do you think its suitable?
Yes should be but would need a drawing of the xover to see what tricks like EQ spacing and notch etc and reverse engineer it to active.
the box is a sealed design, small and heavily damped, little tuning involved I expect..
It will just depend on the amount of series resistance to the woofers only for box tuning and high frequency impedance of the woofer as to how perfect the xover works if the woofers have no zobel across them.
after reading your answers I am tempted to just build it all running off a single 500va transformer.
Yep would work fine but is still probably overkill but that is also fine :-)
if i were to try more caps how should they be added, some physically close to each channel? or does it not matter assuming wiring is good?
Usually a bank close to the rectifier then every amp star powered from there.
the speakers are only about 2m apart anyway with the amp in between... probably will always be something similar... so speaker cabling isn't a big issue i wouldn't think...
No not at all.
Joe :-)
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greenmanhumming
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Re: JLM 60Watt Power AMP Kit Now Available

Post by greenmanhumming »

well I've ordered the AMP kits I need.

if I run 3 amps per channel off the same input, (for interim passive triamping) that will parallel the input impedance, is this a problem?

should I increase an input resistor? from 10k to 20 or 30k?

crossover deconstruction coming soon

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