Thanks for your thoughts. I have a couple of large 24V transformers I can build with but I think I prefer regulation, which generates heat. The open frame linear regulated supplies I have seem to be very quiet but runs hot with the dissipation. Within reason I am actually more concerned about heat and space than costs and hence the thought about SMPS. What I do not know is how bad SMPSs.
Gentlemen,
My version of the JLH69 is the one incorporating the 1970 revision, that is, the bootstrap is replaced with a CCS. The power supply is nothing special, a 120VA EI transformer and an LM317 regulator with a current boosting MJ15004. The output transistors are MJ15003's. With their lowish fT, I believe the stability issues are solved without the need for extra capacitors here and there.
What I would like to say is that this little amp is one of the most silent amps I have ever heard, DIY or commercial.With no signal present and input NOT shorted, putting my ear
right next to the cone, I can hear NOTHING, absolute silence. So, don't go to extreme measures on this respect.
Mine is running at 30V and Iq=2.0 amps. I remember reading somewhere that the output
current from this amp is about 1.5 Iq. , which makes it somewhere around 3 amps in my case.Since the max Vout is around 15 V, the amp will run at full power till the load drops to 5 ohms. The above calculations are only approximate but goes to show that a nominal 8R speaker, if remains over 5R throughout the spectrum, will be happily driven to full power at an Iq of 2.0 amps. Putting the bias to 4.0 amps will definitely stress the output transistors beyond their limits.
Happy soldering
My version of the JLH69 is the one incorporating the 1970 revision, that is, the bootstrap is replaced with a CCS. The power supply is nothing special, a 120VA EI transformer and an LM317 regulator with a current boosting MJ15004. The output transistors are MJ15003's. With their lowish fT, I believe the stability issues are solved without the need for extra capacitors here and there.
What I would like to say is that this little amp is one of the most silent amps I have ever heard, DIY or commercial.With no signal present and input NOT shorted, putting my ear
right next to the cone, I can hear NOTHING, absolute silence. So, don't go to extreme measures on this respect.
Mine is running at 30V and Iq=2.0 amps. I remember reading somewhere that the output
current from this amp is about 1.5 Iq. , which makes it somewhere around 3 amps in my case.Since the max Vout is around 15 V, the amp will run at full power till the load drops to 5 ohms. The above calculations are only approximate but goes to show that a nominal 8R speaker, if remains over 5R throughout the spectrum, will be happily driven to full power at an Iq of 2.0 amps. Putting the bias to 4.0 amps will definitely stress the output transistors beyond their limits.
Happy soldering
Hello Selim,
Point well taken. However, even though my amp is trying to drive the tweeter panel only of my Martin Logan Quest Z, the impedance falls below 2 ohms at 16kHz and essentially behaves like a capacitor so current capability is important.
I am running 2 pairs of MJ15024s per channel and am in the process of improving the heat sinking in addition to adding another pair of MJ15024. Hopefully this can lessen the strain on the devices even if I up the idle current.
Ray
Just received two completed boards...
I've been interested in the JLH 1969 Class A amp for decades. My old AR amp, bought new 12/68, had adjustable center point and bias for each transformer coupled driver and their darlington output stage. Drifting off center and bias was finally solved with fixed selected resistor values. Prior to this, on a hot summer day with no A/C, the amp would leave Class B heading for Class A... cook time! I eventually took it out of service when I got a deal on a Philips FA-50 to go with my CD-960 CDP, the AR amp's PS caps also having dried out. In a move three years ago, after thirty-six years in my previous house, we moved to a garden home with a two car garage. I left a bunch of parts & junk boxes - and the AR amp - there. Dumb. Transformers, heat sinks, half built amps and SW receivers literally abandoned.
I started repairing hifi equipment in the sixties, so I went thru my parts spares and the once great stash was down - 0 2N1711's & 2N2907's - and only a lone 2N3055 - and my caps are old - last purchased were bought in the early eighties. With the usual shock of trying to find parts, I elected to try evil-bay and found an assembled board with Motorola semiconductors for $18.80 shipped from Hong Kong... sounded like a deal to me (Vendor: 'ayanhu81'.). I paid for it Sept 6 - it arrived Sept 19. Actually, I should say 'they' arrived - I received two amps - the ordered one and a mirror image! They haven't replied to my e-mail, but I gave them a positive rating just the same.
Now I can ask my questions!
1. Anyone have experience with this board?
2. I only need 5W of clean power for my Klipsch KB-15 speaker in a mono application. I want to use the xfmr from a commercial 13.6V 3A regulated PS - should be 15V secondary - yielding~20V from a br rect and decent sized cap under a 1.2A Iq... will this suffice?
3. Can I mount the Al L bracket to a 5" x 7" x 3" Al chassis for it's heat sink?
Thanks for your help!
John
I've been interested in the JLH 1969 Class A amp for decades. My old AR amp, bought new 12/68, had adjustable center point and bias for each transformer coupled driver and their darlington output stage. Drifting off center and bias was finally solved with fixed selected resistor values. Prior to this, on a hot summer day with no A/C, the amp would leave Class B heading for Class A... cook time! I eventually took it out of service when I got a deal on a Philips FA-50 to go with my CD-960 CDP, the AR amp's PS caps also having dried out. In a move three years ago, after thirty-six years in my previous house, we moved to a garden home with a two car garage. I left a bunch of parts & junk boxes - and the AR amp - there. Dumb. Transformers, heat sinks, half built amps and SW receivers literally abandoned.
I started repairing hifi equipment in the sixties, so I went thru my parts spares and the once great stash was down - 0 2N1711's & 2N2907's - and only a lone 2N3055 - and my caps are old - last purchased were bought in the early eighties. With the usual shock of trying to find parts, I elected to try evil-bay and found an assembled board with Motorola semiconductors for $18.80 shipped from Hong Kong... sounded like a deal to me (Vendor: 'ayanhu81'.). I paid for it Sept 6 - it arrived Sept 19. Actually, I should say 'they' arrived - I received two amps - the ordered one and a mirror image! They haven't replied to my e-mail, but I gave them a positive rating just the same.
Now I can ask my questions!
1. Anyone have experience with this board?
2. I only need 5W of clean power for my Klipsch KB-15 speaker in a mono application. I want to use the xfmr from a commercial 13.6V 3A regulated PS - should be 15V secondary - yielding~20V from a br rect and decent sized cap under a 1.2A Iq... will this suffice?
3. Can I mount the Al L bracket to a 5" x 7" x 3" Al chassis for it's heat sink?
Thanks for your help!
John
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Joined 2009
Paid Member
My JLH hasn't really be used since I built it - took years by the way! I find the sound too dark and rolled off. I'm using ancient single diffusion 2N3055H transistors for the outputs. I was warned not to, that it would be better to start with better output devices. I didn't listen. And as predicted, I don't listen to the amp at all. So I would suggest to others it's worth using better devices. Sanken have a very good reputation. I'm not sure why the 2SC2911 are not sounding good, it's possible that you have a little bit parasitic oscillation and you could fix this as JLH once suggested with resistor+cap shunt compensation - it's in one of the articles, can't remember the details right off the top.
There is no point using high speed output transistors that were intended for the switching demands of class AB amplifiers, in a class A amplifier. Why do you think old parts like 2N3055 are still specified and preferred in JLH amplifiers?
People who are new to audio electronics, often make the mistake of thinking you can just swap semis at will to high speed (high Ft) transistor types and all audio qualities will be improved. What is missing there is an understanding of how this also reduces stability margins. The JLH class A amplifiers are simple and have no preventive measures that would damp any oscillation, which is likely what you perceive as metallic sound quality when 2SC2922 transistors were fitted.
I'll bet that the specified types sound even better but if you buy your parts from Chinese sources, who knows what silicon you really have fitted anyway.
People who are new to audio electronics, often make the mistake of thinking you can just swap semis at will to high speed (high Ft) transistor types and all audio qualities will be improved. What is missing there is an understanding of how this also reduces stability margins. The JLH class A amplifiers are simple and have no preventive measures that would damp any oscillation, which is likely what you perceive as metallic sound quality when 2SC2922 transistors were fitted.
I'll bet that the specified types sound even better but if you buy your parts from Chinese sources, who knows what silicon you really have fitted anyway.
Member
Joined 2009
Paid Member
Gannaji - Yup, that's the one. Shows the optional compensation R + C. Something like this, done right, could allow use of faster transistors.
Ian - I think it's a basket of attributes we're looking for here in order to achieve better performance. Probably the key one is that modern devices have lower beta-droop in a design that JLH has shown produces less distortion when low beta is avoided.
Ian - I think it's a basket of attributes we're looking for here in order to achieve better performance. Probably the key one is that modern devices have lower beta-droop in a design that JLH has shown produces less distortion when low beta is avoided.