JLM 60Watt Power AMP Kit

[Joe Malone]

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

The Balanced input impedance when not using the volume control feature for a 60W Amp is 40k so shouldn't have to change anything for now. Unbalanced input is half the balanced input impedance.

[greenmanhumming]

ah ok, it's only 10k with the pot in place.... great..

[Joe Malone]

ah ok, it's only 10k with the pot in place.... great..

With Pot Minimum is 20k balanced input impedance

[greenmanhumming]

I'm going the 3way monoblock way... for now anyway, keeping it as modular as possible, so I can change to 3 stereo amps if I want later

I have one monoblock working so far. just tri-amped passive for now, active crossovers are the next stage

if i want to adjust the fixed gain of each amp to balance the driver levels, what value resistor should I put across the pot terminals to achieve a 1dB gain reduction? 2dB? How do I work this out?

EDIT both channels now working and sounding great, will listen for a while then start tweaking, and working out about going to active crossovers

[greenmanhumming]

heres the schem of my Linn Kabers' passive crossovers.

any thoughts about designing active crossovers to replace this?



drivers all nominal 8ohms

[Joe Malone]

Ok looks like the tweeter is 8ohm by the Zobel values?

Are the 2 woofers 8ohms each?

I presume so as they are both in parallel at low frequency.

[greenmanhumming]

yep drivers all nominally 8 ohms, woofers measure about 6.2ohms DC and tweeter about 5.6ohms DC.

midbass and bass drivers identical as far as i can tell.

[Joe Malone]

Passive filter into pure 8ohm loads
I will see if I can emulate these with filters on the inputs or vol position of the AMP kits

[greenmanhumming]

Thanks for working that out! It would be great if it can be done that simply.

Are there advantages to building a separate board with opamp driven active filters? Or is it really unneccesary?

The existing crossover may be improvable on... It's not the last version linn did , and IMHO the tweeter could come down another 1 dB or so.

The bass driver could prob come up by 1db too, but this may happen naturally when passive losses eliminated.

All that said even with the passive triamping this is an extremely clear and revealing system!


What is the 6r8 in series with the 500uf about? That's the bit that most confuses me.

[Joe Malone]

Thanks for working that out! It would be great if it can be done that simply.

Are there advantages to building a separate board with opamp driven active filters? Or is it really unneccesary?

Separate active Xover makes it easy to do the 18dB slope on the tweeter and eliminates inductors. So makes its easier overall.

The existing crossover may be improvable on... It's not the last version linn did , and IMHO the tweeter could come down another 1 dB or so.

The bass driver could prob come up by 1db too, but this may happen naturally when passive losses eliminated.

All that said even with the passive triamping this is an extremely clear and revealing system!

Level setting is easy with multiple amps. And the balance when driven direct from amps is different due to damping once any of the old passive xover inductors are eliminated which can also change the box tuning as well. Do you have any info on the later Xovers?

What is the 6r8 in series with the 500uf about? That's the bit that most confuses me.

Yes this zobel is strange as it is so low but it will be trying to smooth the impedance to be flat for the box/speaker resonance which is a bell shape so impossible to smooth. Normally the Zobel is aimed at the raising impedance of the woofer above 100Hz or so you would think the mid/bass would have a zobel instead of the bass driver.

Are the bass drivers setup D'Appolito (symmetrical above and below the tweeter) or not? Is the box ported or not?

Are you running the inputs to amps balanced or unbalanced?

[greenmanhumming]

Sealed box, rather small internal space, v v heavy and well braced, d'appolito config, no info on later xover, drivers also slightly changed so who knows? Running inputs balanced. Scuse brevity posting from phone

[greenmanhumming]

just in case it helps heres some pics:

the bass driver xover:


the mid and treble xover:


the front of the speaker:


one more question, won't the 18dB slope on the tweeter put it out of phase with the woofers which have 12dB slopes?

[greenmanhumming]

BTW, I have a passive balanced volume control that I use in a switching box to select which source to monitor on which monitors.

It is a stepped attenuator, and its balanced by putting a conventional vol control in each side of the balanced signal, so for stereo its a 4 gang attenuator.

it works fine with my mackie monitors but with other amps i get a slight hum unless the attenuator is turned all the way up.

whats going on here?

I see theres a schem on the JLM site that shows a passive volume control that works by phase cancellation between the 2 sides of the balanced signal, is this more correct? what are the pros and cons of each way of doing it? is this what the vol control on an amp kit does? (would still like to see the AMP schem)

[greenmanhumming]

bump!

I'd love some input on some of the issues above

* humming volume control inline, whats the best way to do a remote inloine balanced volume control?

* mixing 18db and 12 db slopes doesnt this cause phase issues?

* any more input at all on the best way to build a active crossover for these speakers with the 3 amp kits per speaker...

[Joe Malone]

bump!

I'd love some input on some of the issues above

* humming volume control inline, whats the best way to do a remote inline balanced volume control?

The quad pot volume has lots of problems with matching if using a actual pot due to the tolerance of usually 20% between pots. And if it is connected to ground as well there are other problems. I prefer the single shorting pot as the balanced impedance's and output level can be 1% even with a normal pot. Also a dual pot can control stereo.

* mixing 18db and 12 db slopes doesn't this cause phase issues?

Yes but if the drivers are are not time aligned the phase difference maybe a attempt to correct the mechanical phase difference.

* any more input at all on the best way to build a active crossover for these speakers with the 3 amp kits per speaker...

This is the simulation I did over Xmas for doing the Xover and bass boosting around the amp modules with both woofers used as mid/woofers.
Green and yellow are the active responses over the blue and red original passive curves


Doing the xover around the amps loses the volume function and needs to be run by a low impedance so a passive volume control is not a good idea. Best would be using 2 of our dINgO Pups with a stereo volume connected to the unbal in and out.