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Author Topic: JLH 1969  (Read 880 times)

sevrien

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JLH 1969
« on: February 21, 2020, 06:38:44 AM »
Hello,

I'd like to build a small 10 to 15 Watt practice combo or head.
For the power amp-section I've found this simple and cheap kit.

https://nl.aliexpress.com/item/32950118672.html?spm=a2g0o.productlist.0.0.4f645da4QxriVU&algo_pvid=51bd4610-c353-4b6e-9f48-8c7d12448680&algo_expid=51bd4610-c353-4b6e-9f48-8c7d12448680-0&btsid=0ab50f4415822832829347385ea825&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_

Has anyone ever tried to use this one?
Any doubts or things to keep in mind?
I know its class A and it gets hot but it's supposed to deliver a quite warm tube-like sound.

I haven't decided yet what preamp goes in front. maybe I make it modular so I can slide in different preamps depending on the mood I'm in.

Greetings
Sevrien

Loudthud

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Re: JLH 1969
« Reply #1 on: February 21, 2020, 09:01:13 AM »
Not worth the trouble IMHO. When audiophiles talk about warm tube sound, it's not the same as a tube guitar amp.

Check this thread: https://www.ssguitar.com/index.php?topic=3747.0

Scroll down to my post about half way down the first page. You can use a 24V filament transformer or even a 24VAC 40VA wall wart for a power supply.
« Last Edit: February 22, 2020, 06:22:01 AM by Loudthud »

Loudthud

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Re: JLH 1969
« Reply #2 on: February 22, 2020, 06:41:15 AM »
Kevin O'Connor devotes a chapter to the JLH in his book, Tonnes of Tone. He runs his version on slightly over 50V which should give about 25W into 8 Ohms. He calls it the "Hood" amp. The heatsink and power transformer run very hot is his design and he doesn't give you a way to adjust the idle current. It really needs twice the heatsink and a bigger transformer.

Another short coming is he doesn't include mixed mode feedback. It would only take one resistor and would go a long way to make the amp sound more tube like.

If you want something solid state that sounds like tubes, look into the designs of Nelson Pass. He's on diyAudio and has a couple of websites. You'll still need a big heatsink.

phatt

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Re: JLH 1969
« Reply #3 on: February 22, 2020, 07:07:28 AM »

I Agree with *Loudthud* :tu:
I would simply add that the term Class A is kinda meaningless for Guitar amps,, Leave that silly stuff for the HiFi cork sniffers.
Most of all the greatest guitar sounds you have ever heard were played on mainly Class A/B amps,,, NOT Class A.

With SS Poweramps there is only a small benefit to be gained by trying to emulate Tubey sound. Yes there are tricks like current FB which help. Peavey uses the term TransTube,,,, But I must be the worlds worse Cork sniffer  :lmao: 
Cause I've never heard anything that even comes close to tube sound from the Peaveys I've played. xP
Transtube only seems to work at loud SPL but at lower levels it's snake oils to my ears.

The best you can do with a SS rig is trick up the preamps and just build a simple SS Poweramp.
Chip amps are dead easy to assemble and work very well,, especially if you are new to this amp building fun game.  Spend time reading up on the subject before you commit to building.

Plenty here to wet your appetite;
https://sound-au.com/projects-0.htm#pwr

Well written and with the novice in mind.
You can purchase pcbs from him with backup help if needed.

If this is your first build attempt then I'd go with Project 19, Single Chip 50W Power Amplifier.
It's for LM3876 or LM3886.

Tons of good info on this site.
Rod E has helped me with a few issues even though I have not purchased any projects. :tu:
Phil

Loudthud

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Re: JLH 1969
« Reply #4 on: February 22, 2020, 09:29:06 AM »
I built a JLH last week and here's a THD graph I plotted. The power supply is only about 30V so power is about 10W at 8 Ohm. Notice how THD rises slowly and then takes a turn about 10W at .5% THD. This is where hard clipping starts to occur. The heatsink is 1 degree per Watt runs 55 degrees C. Too hot to keep your hand on.

Contrast the above to the Nelson Pass design called the ACA. This is a little amp that does about 10W at 4 Ohms. Two channels are bridged to give about 20W at 8 Ohms. Notice how THD is rising slowly but doesn't take the turn until you get near 20W at 5% THD. Much more tube like. The ACA kits you can find at diyAudio run on a 24V switching supply so you don't have to mess with mains wiring.

Loudthud

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Re: JLH 1969
« Reply #5 on: February 22, 2020, 10:46:04 AM »
For comparison below is a THD plot of a TDA2030 chip amp and the MC12DAR that I linked above.

Note the Chipamp has low THD for much of the graph, then rises steeply at the right where clipping starts to occur.

The MC12DAR has no global AC feedback to it's THD is higher but it has a lot more gain. The THD rises gently and reaches several percent before clipping occurs.
« Last Edit: February 22, 2020, 10:51:16 AM by Loudthud »

phatt

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Re: JLH 1969
« Reply #6 on: February 24, 2020, 07:22:34 AM »
Hi Loudthudsy,
                     I tried to search for that MC12DAR circuit but got nothing meaningful.
I'm interested to try it out as long as it does not need unobtainium reverse wound diodes. :lmao:
I think my problem with trying to get SS PwrAmps to respond like a Valve PwrStage is that
Even if one can reproduce a tuby SS Pwr Stage one is still stuck with the same limitation as A valve pwr stage. That being, they only sound good when they are cranked up. I noticed this with some of the Peavy gear.
So having built quite a few amps now both valve and SS I just use pedals for all the dirt tricks and the amp is a clean simple SS PwrAmp.
I also have built a few ReAmp systems for myself and a couple of local players with quite stunning results.
A small 10w pp Valve amp driving a resistive load and line out to a clean all SS second amp.
This removes the limitation of having to play loud to get the magic OD of Valve pwramp.
So from bedroom to gig SPL's the sound/tone/dynamics are much the same.
BTW my resistive load is NOT the common 8 Ohm load on 8 Ohm tap which as you may already know kills the magic.
Thanks for posting the plots, very helpful.
Phil.

Loudthud

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Re: JLH 1969
« Reply #7 on: February 25, 2020, 07:19:35 AM »
Phatt,

The MC12DAR was posted about half way down the first page in this thread.

Link: https://www.ssguitar.com/index.php?topic=3747.0

This is just a simplification of the circuit I actually use, it's intended for beginners. If you can't obtain the TIP darlingtons, you can try any other big darlington, but some changes to the circuit may be needed. The two 2.2K resistors may need to be adjusted to get full output. Everyone thinks an emitter follower should have a low output impedance. In actuality, the output impedance is the impedance at the input, divided by beta. On my prototype, the output impedance IIRC is about 4 ohms (measured by the delta E over delta I method using 4 and 8 Ohms). So adjust the 2.2K resistors up or down so you can obtain rail to rail output, but don't go lower.

If there is any interest, I could do a new layout, maybe even order some PCBs.

I think you lose too much when you are not driving a power amp to the rails. It kills the feel of the amp.


phatt

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Re: JLH 1969
« Reply #8 on: February 27, 2020, 10:33:16 PM »
@ Loudthud,
Oh thanks for that,, I just realised that I was not logged on so did not notice the schematic,, durh me. :duh
well as luck would have it i have a pair of TIP142/147 and a few 2N7000.
Not about to build, just wanted to Bboard the circuit and see how it performs.
I'm not expecting the same Grind I get from my little 10 watt valve amps but if it gives a nice little rattle I might pursue it further.
I agree, pwramps sound good when driven hard but then I'm back to the SPL issue, even a 10Watt amp is often too loud for home practice. :-\
I assume there is no short circuit protection and only about 20 watt output??
thanks for your help, Phil.

Loudthud

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Re: JLH 1969
« Reply #9 on: February 28, 2020, 09:08:37 AM »
Those TIP parts don't have a very good SOA (Safe Operating Area) and the ON Semi parts are a little better than ST. So a safe limit for the rail is 33V or what you get with a 24VAC transformer with a bridge-cap type filter. But you can use a 4 Ohm load and slam the rails all day long if your heatsink is big enough.

A 40 VA wall wart transformer should get the job done and be safe for amateur builders. If possible, get one with a safety ground, it will help keep the hum low. In the USA you can use MG Electronics MGT2450. Be aware that these units have an internal fuse, if you blow it, you have to replace the transformer :(  That should get you close to 15W at 8 Ohms or 25W at 4 Ohms.

I'll experiment with short circuit protection and update the schematic. It'll take 6 or 8 diodes.

Playing at bedroom levels is always going to be a compromise. You never get really good tone. A simulated speaker load and headphones might get you close.

g1

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Re: JLH 1969
« Reply #10 on: February 28, 2020, 09:20:15 PM »
I think you lose too much when you are not driving a power amp to the rails. It kills the feel of the amp.
Playing at bedroom levels is always going to be a compromise. You never get really good tone. A simulated speaker load and headphones might get you close.
This seems as good a place to ask as any.  Attached circuit has some similarities to the TIP example.  The question came up elsewhere, why are they using power resistors at the speaker for attenuation rather than a master volume pot as they used in their larger power TS models?  Someone suggested that they wanted to keep the power section 'working harder', but I noted that they are putting the resistors in series with the speaker rather than a series parallel arrangement.

Is there any advantage here to doing it this way rather than using a (cheaper) switchable attenuator at the power amp input? (people were commenting that the amp sounded surprisingly good for a practice amp)

Loudthud

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Re: JLH 1969
« Reply #11 on: February 29, 2020, 08:33:26 AM »
A low damping factor just sounds better. Those attenuators that are basically just a Voltage divider pot across the output don't sound very good until you add series resistance to the speaker. A solid state power amp doesn't require a load, and the argument can be made that it sounds better without it. My latest Proof of Concept experiment compared a switching power supply against a soft analog power supply. The switching supply didn't sound that bad. Different, but not bad.