Yes, and this has been discussed in this long thread too.
Especially for BJTs where current is needed, and in the MOSFET variant enough current is needed in the VAS to be able to drive the lower FET at full current, which is more difficult with source resistors and relatively low gate to-ground resistors (R9). The variable resistor for the current setting needs to be the upper resistor rather than R8 so that R8 can be set to the same as R9. For a MOSFET circuit the use of a CCS will be very limiting due to the Vg needed (about 3-4 V) as you say, so a bootstrap is indeed preferred.
The JLH original circuit can be optimised for lower distortion than originally published using modern linear and high gain output transistors, which makes it better I would suggest. I did not like the sound of the original when handling complex music although on single instruments/voices it was adequate. The performance figures originally published were largely relating to 15 ohm speakers and pretty much worsened for 8 ohm.
Thanks a lot J! Just what I needed and suspected, it’s all about sensitivity (and impedance).Hello Chumingo,
I hope you don't mind me answering your question with a link to an explanation, briefly speaker Watts are meaningless what you need to find out is their db rating. For the full story and the simple maths please go to this page https://jlh1969classa.blogspot.com/p/watts.html it's too much to copy and paste here.
You will find that you are listening to micro Watts!Cheers -J
More on speakers… I’m after a pair of old Braun boxes (L470) that I like, but don’t cover deep lows (13cm woofer). Is it doable to add a passive mono Sub to the high-level output of the amp, ie. mixing the stereo lines with resistors (to avoid crosstalk on the main speakers), followed by a passive LPF for the Sub? Or the Sub needs its own dedicated amp?
I hope the question is clear. My alternative is to add a parallel active Sub (with whatever amp it comes) or simple get bigger speakers. Any suggestions are welcomed!
I came across a more recent update. The use of constant current sources is interesting.
https://startfetch.com/jlh/jlh-2003-amplifier.pdf
Last update seems to be from 2013
I can't find any references to a LH mosfet design other than a rather complex one from the 80's.
https://startfetch.com/jlh/jlh-2003-amplifier.pdf
Last update seems to be from 2013
I can't find any references to a LH mosfet design other than a rather complex one from the 80's.
Braun L610,
2-Wege-Box mit 18 cm Tief-Mittelton Lautsprecher und 2,5 cm Kalotten-Hochtöner.
Belastbarkeit 35 W nach DIN. Impedanz 4 Ohm. Nettovolumen 15 Liter.
Übertragungsbereich: 30...25000 Hz.
https://www.radiomuseum.org/r/braun_hifi_lautsprecher_einheit_l610.html
The amplifier must be rated at 4 ohms.
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"...modifications carried out during the past year by one constructor – Tim Andrew...Before Tim first contacted me, he had built a kit version of the 1996 design, which he had subsequently upgraded with higher quality components. Though Tim was happy with the results, he was keen to see if further improvements could be made to the sound quality and I was pleased to be able to suggest various circuit modifications..."
https://www.diyaudio.com/community/threads/jlh2003.299690/page-2
https://www.diyaudio.com/community/threads/geoff-mosss-jlh-2005-design-files.369279/page-3
There are more posts about this mod. Looking for...
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I'm a newbie calculating power requirements for speakers, but ohm's law tells me 4ohms at 11V mean 2.75A load to amp. I assume 11V bias midrail because the AC transformer input of the JLH I would be using is 22V. That's 30W, a lot for the it.
I suppose one never wants the speakers' full power, but I'm pretty stuck with these calculations. I read that output impedance of the JLH is close 0.5ohms, there shouldn't be a problem there. Supposing I want the speakers to play 10W max., I would trimm for example the amp to a max. of 100mA, those 11V would become 0.4V on the speakers load, right? A little low but usable?
In other words, can the 10W JLH drive 4ohm speakers like the Braun L610?
And are the speakers' ohms calculated anyways, if there are a tweeter and a woofer actually? Are both drivers 8ohm and in parallel make for 4ohm?
Sorry, seems like many questions. Please some light.
Thanks a lot OldDIY. In practice I would only have to reduce R1 to 47ohm, since R2 is a trimpot in my case. Besides the caps... fortunately I have all parts on the shelf.
But that would be for 3ohms. Are the mods also adequate for 4ohms? Also, my power supply is 22V
bit higher than the 17V suggested for 3ohms impedance, should I do something about it?What OldDIY showed is JLH's table for different impedances. That means you could adjust for 4 ohms by increasing the resistors for 4 ohms or you could run the 3 ohm version. What is a concern is that you should be prepared to adjust the resistors to achieve the bias current which is needed. If you run the 3 ohm component values from 22V the current will increase, as an estimate in proportion (22/17) which means that the power dissipated by the transistors will be higher, which means you will need to make sure that your heatsink is capable of dissipating the power.
Your power supply voltage may droop with an increased load somewhat. I would not be particularly concerned at running a 3 ohm set-up with 4 ohm loads, the load impedances of a speaker can vary with frequency as OldDIY shows. But you will need to make sure that the transistors can be kept adequately cooled. You could instead use a stabiliser on your power supply. Not usually thought to be necessary for a BJT JLH (the MOSFET circuit is quite different especially if running in AB as Old DIY suggests. I would run that in Class A too, but trawling back a few decades shows that a current limter on turn on might be a good idea). A stabiliser would need its own (good) heatsink but should benefit in reducing hum.
Your power supply voltage may droop with an increased load somewhat. I would not be particularly concerned at running a 3 ohm set-up with 4 ohm loads, the load impedances of a speaker can vary with frequency as OldDIY shows. But you will need to make sure that the transistors can be kept adequately cooled. You could instead use a stabiliser on your power supply. Not usually thought to be necessary for a BJT JLH (the MOSFET circuit is quite different especially if running in AB as Old DIY suggests. I would run that in Class A too, but trawling back a few decades shows that a current limter on turn on might be a good idea). A stabiliser would need its own (good) heatsink but should benefit in reducing hum.
Doesn't it make sense to increase R1 a tiny bit (say to 52ohm) and go a bit under 2A (ie. 1.8A), to make for a "4ohm version" and alleviate the heatsinks? Just reckoning here.
The current is regulated by the trimpot on R2, right? And to measure it the easiest way is testing the current between R1 and R2? How do you usually find out the current 'sweetspot' for a particular load?
I highly appreciate both table @OldDIY and clarification before @john_ellis. Even though I bet these questions have been asked before, it is really difficult to find them.
You can do that of course. That would be in line with the proposal JLH made for different speaker impedances.
The only point I was making was that speaker impedances might vary and go below the nominal at some frequencies, so allowing a little more current will avoid clipping due to insufficient current, or as some might suggest "going into Class AB", which I would try to avoid.
The trade-off is that if you keep a higher current, the power dissipation increases in your transistors, but you wil be able to get a little more power out of the amplifier. But if you dont drive your amplifier to the limit, you can run with reduced current. Not a big problem.
I take it you are working with the original JLH circuit? Depending on the gain droop of the transistors you use then you may want to run at slightly higher than the calculated current to allow for gain droop. That means looking up the datasheets of the transistors you plan to use, another reason you may want to think about slightly higher than the nominal current. Some devices are better than others. It's a point which affected older transistors more, though.
The only point I was making was that speaker impedances might vary and go below the nominal at some frequencies, so allowing a little more current will avoid clipping due to insufficient current, or as some might suggest "going into Class AB", which I would try to avoid.
The trade-off is that if you keep a higher current, the power dissipation increases in your transistors, but you wil be able to get a little more power out of the amplifier. But if you dont drive your amplifier to the limit, you can run with reduced current. Not a big problem.
I take it you are working with the original JLH circuit? Depending on the gain droop of the transistors you use then you may want to run at slightly higher than the calculated current to allow for gain droop. That means looking up the datasheets of the transistors you plan to use, another reason you may want to think about slightly higher than the nominal current. Some devices are better than others. It's a point which affected older transistors more, though.
Actually the build I'll be working on seems to be a slight mod from the original JLH. I've heard it comes with a PNP output stage, TO-3 transistors are supposedly labeled MJ2955, but let's see what comes. Unfortunately I didn't have the time to start building my own from scratch, since I'm moving from flat recently and many other to-do's ahead so I tried to advance as much as I could. This is it, it should arrive over the next days.
It also includes a capacitance multiplier on the power supply, which seems to take care of ripple and hum. I guess that's the stabilizer you meant before @john_ellis?
BTW. What does Vin(rms) on the table shared by @OldDIY exactly mean? Is it the max. input voltage that the amp's input stage can handle before clipping? I see on the table that the 4ohm version is only about 0.4vrms.