I recently come way from a charity shop with a pile of equipment which was only for sale as a job lot, after I spotted an interesting looking amplifier at the back.
It turned out to be a Hart Linsley Hood kit - Stereo 300 Series - and came with a pile of documentation too. It sounds fantastic, very powerful and controlled, but has what seems to me to be slightly excessive hiss. This is not affected by the volume control and is the same for all inputs, connected or not, shorted or open. It varies with the treble control, and doesn't affect the overall sound when playing music, but just seems a bit intrusive when just idling.
I was wondering if any experts could see anything in the design which would cause this; could it be ageing components or is it something I just have to live with?
It turned out to be a Hart Linsley Hood kit - Stereo 300 Series - and came with a pile of documentation too. It sounds fantastic, very powerful and controlled, but has what seems to me to be slightly excessive hiss. This is not affected by the volume control and is the same for all inputs, connected or not, shorted or open. It varies with the treble control, and doesn't affect the overall sound when playing music, but just seems a bit intrusive when just idling.
I was wondering if any experts could see anything in the design which would cause this; could it be ageing components or is it something I just have to live with?
It is conceivable an oscillation of the power amplifier: CFP, Sziklai. A small capacitor, 100 pF - 470 pF, between collector and base of the power transistors Tr7, Tr8 should help. Sometimes it is necessary Tr5, Tr6 as well to make happy.
It’s possible the hiss is inherent in the design.
I suggest trying to determine where the hiss originates. As an experiment, at the opamp, install a short between pins 2 and 6. This sets opamp to unity gain, so no signal. Is hiss now acceptable?
What is opamp type?
I suggest trying to determine where the hiss originates. As an experiment, at the opamp, install a short between pins 2 and 6. This sets opamp to unity gain, so no signal. Is hiss now acceptable?
What is opamp type?
op amp is Motorola C1741CP. Shorting pins 2 and 6 kills the hiss completely. What does this reveal? (apart from my lack of knowledge, of course)
Mainly, it absolves the amplifier stage of any significant contribution to the hiss problem, so that's good news.
The C1741 is a Motorola version of a LM741 opamp, an ancient and not very good for audio work. Another issue is that relatively large resistors are used on the tone control, making is even more susceptible to bias current noise in the opamp. Swapping it for a TL071 may do wonders.
The C1741 is a Motorola version of a LM741 opamp, an ancient and not very good for audio work. Another issue is that relatively large resistors are used on the tone control, making is even more susceptible to bias current noise in the opamp. Swapping it for a TL071 may do wonders.
Swapping the opamp is probably the easiest remedy. It will almost certainly improve hiss behavior, but you may still be dissatisfied with remaining hiss, especially if you use treble boost.
Another aggravating performance issue is the design of the balance control. It boosts gain and noise of the tone control section by about 6dB. You can experiment with a short across R24; you will lose the balance control feature, but you'll get a feel for the impact. Of course an upgraded opamp will help this issue also.
Still other mods are possible if you're adventurous, but they would entail adding a small circuit board to add an additional opamp; that would allow the balance control to be placed in front of the tone control section.
Another aggravating performance issue is the design of the balance control. It boosts gain and noise of the tone control section by about 6dB. You can experiment with a short across R24; you will lose the balance control feature, but you'll get a feel for the impact. Of course an upgraded opamp will help this issue also.
Still other mods are possible if you're adventurous, but they would entail adding a small circuit board to add an additional opamp; that would allow the balance control to be placed in front of the tone control section.
Thank you. I'll swap them for TL071s as they're only 90p and won't take long, so nothing to lose!
Sorry, posts crossed over. I will try shorting R24 - I didn't like the balance control, and wouldn't ever use it anyway, and the gain seems rather high in any case. I'll try the opamp swap if I can get a couple tomorrow, and lose the balance control too. Many thanks!
I am sure you will get much better results with the TL071 The 741 were never much use re the noise factor you can of course get even lower noise op amps these days
Best of luck
Trev
Best of luck
Trev
I'll take a few more when it's back together. I'll try to get the ICs today and get it done.
I also have copies of the original articles from HiFi News with diagrams and PCB layouts which I can scan and post if you're interested (and it doesn't infringe too many copyrights....)
already, thank you for the photos, I'm jealous of your find!
then, you saw that you have mosfets at the output?!
then, you saw that you have mosfets at the output?!
My two cents . Hiss you hear is because preamplifier , tone control ,is connected to power amplifier , and opamp noise , whatever low or high it would be , is always amplified by power amplifier. If you wanna , can try to move volume control potentiometer to power amplifier input. Thus you will reduce hiss at same proportion with signal level. At max volume position, you will have same hiss as you have now. Also current volume control design explains why hiss didn't change while adjusting volume or shorting inputs.
Thanks! To be honest, the level of hiss is now so low that it's perfectly acceptable, and is scarcely any louder at maximum volume (which I am unlikely ever to use) I would like to keep the amp as close to original as possible, so for now I think I'll just use it as it is, but, if I use it with different speakers or in a setup where the hiss is intrusive, I will try that.
What a great find its a real looker I remember the articles well I am a little jealous or should I say envious
Trev
Trev
I've just posted the articles on a separate thread 'JLH 30 watt amplifier articles'
It was a lucky find, and included a JLH tuner and cassette deck too. The tuner seems fine (I haven't yet tested it with a good aerial) and I tidied up the display with a clear perspex screen. The cassette deck may not be finished (although there was a cassette in it) and I think I will probably pass them both on as I won't use the tuner and already have a cassette deck for archiving.
It was a lucky find, and included a JLH tuner and cassette deck too. The tuner seems fine (I haven't yet tested it with a good aerial) and I tidied up the display with a clear perspex screen. The cassette deck may not be finished (although there was a cassette in it) and I think I will probably pass them both on as I won't use the tuner and already have a cassette deck for archiving.