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?
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.
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.