When comparing HCA-3500 to JC-1, I noticed that the Zener diodes for the MOSFET driving transistors are different. In HCA-3500, 30V zener is used, while in JC-1 18V is used. Both are noted as D35/D36 in the schematics.
Any one know the reason ? and how this affects the sound ?
It would be great if John Curl could help understand this.
Thanks,
Ray
Any one know the reason ? and how this affects the sound ?
It would be great if John Curl could help understand this.
Thanks,
Ray
Here are the pictures showing the diodes
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
The schenatic of HCA-3500 shows +/-90V, while the schematic of the JC-1 doesn't show voltage. However, both use the same 33000uF 100V capacitors. I assume the voltages are close to each other, as the JC-1 is rated at higher output power.
In any case, the zener diodes are used to provide the bias of the MOSFET. By using a 18V diode, the R10/R3 should carry a much higher current to compensate the 12V different between the two different diodes. Any explaination ?
That zeners only for protection. They are not conduct in normal daily use...
The value is not very critical until it protect within the maximum values of the mosfets.
Sajti
The value is not very critical until it protect within the maximum values of the mosfets.
Sajti
Another quick quesstion.
John mentioned earlier that the hca-3500 was mistakenly placed too many decoupling capaitors on the power rails, which resulted in unsatisfactory sound. As such, some mods recommend removing some of them. But why too many capacitor could hurt ?
John mentioned earlier that the hca-3500 was mistakenly placed too many decoupling capaitors on the power rails, which resulted in unsatisfactory sound. As such, some mods recommend removing some of them. But why too many capacitor could hurt ?
I can't tell you precisely why, but many caps can be removed from the power supply and the unit will most probably work better. Compare the schematics of the JC-1 and the HCA-3500 and note the caps in the power supply. It is possible that the extra caps, while looking 'impressive' actually add resonances as much as reduce noise.
However this is a minor change. The big problems were the use of an unknown type of feedback resistor, instead of a Holco 47K that was specified in writing, and the leakage of one of the smaller 'housekeeping' power supply transformers that was wired without regard to minimum leakage.
However this is a minor change. The big problems were the use of an unknown type of feedback resistor, instead of a Holco 47K that was specified in writing, and the leakage of one of the smaller 'housekeeping' power supply transformers that was wired without regard to minimum leakage.
Thank you. Glad to hear directly from you !
The resonance explaination makes sense. And also, I have chaged the feedback resistor to Holco.
What's about the "leakage of one of the smaller 'housekeeping' power supply transformers that was wired without regard to minimum leakage"?
Where is that transformer located ? is there a way to improve ?
The resonance explaination makes sense. And also, I have chaged the feedback resistor to Holco.
What's about the "leakage of one of the smaller 'housekeeping' power supply transformers that was wired without regard to minimum leakage"?
Where is that transformer located ? is there a way to improve ?
By the way, I have also jumped the dynamic bias Q35, to make it more like the JC-1. However, I noticed noise increase (mid-frequency hum) due to the high bias current (The voltage across the emmiter resistor is about 20mV). It behaves similar as tube amps. Is it because of the MOSFET transistor ? How do you deal with it in JC-1 ?
The JC-1 have a bias switch. I assume the noise level increases as the switch turned to high bias.
The JC-1 have a bias switch. I assume the noise level increases as the switch turned to high bias.
Also, for the balanced input, a buffer is applied on the negative leg as shown in picture below. Can I bypass it ? what the negative impact ? DC offset issues ?
Sorry for so many questions 🙂
Sorry for so many questions 🙂
An externally hosted image should be here but it was not working when we last tested it.
When the switch is between 1 and 2, the balanced driver is effectively outside the circuitry and totally removing it will not do much.
Sorry I can't help you with the transformer, I never did the mod on the 3500 myself, my associate did it about 15 years ago.
The hum that you are getting is not clear to me. It should not be there, but be careful with grounding the 3rd terminal on the AC line cord. That sometimes creates a ground loop. 20 mV should be OK for biasing.
Sorry I can't help you with the transformer, I never did the mod on the 3500 myself, my associate did it about 15 years ago.
The hum that you are getting is not clear to me. It should not be there, but be careful with grounding the 3rd terminal on the AC line cord. That sometimes creates a ground loop. 20 mV should be OK for biasing.
Hca3500 emitter resistors
Hi
Thinking about replacing these. Any one knows whick ones installed? They are greenish, black striped (non inductive?..)
I looked at Mills mra5, and ohmite audio gold ag5 or ag10(10w). Here is the thing:
Ag10 are 10w,and about 45mm long, so not sure I can squeeze them in. Besides, seems highercwarttage is ok, or am I wrong?
As far as the ag5, they are gone, can get either 0.20 ohm or 0.24 ohm. Seems 0.20 would generate more heat (watt=Volt*volt/resistance), and anyhow using something different than the original 0.22 ohm must have some impact, rebiasing the voltage drop maybe?
The basic question is will I get a noticeable improvement in sound? Is it worth it? Mills or audio gold? 0.20 or 0.24 ohm?
Hi
Thinking about replacing these. Any one knows whick ones installed? They are greenish, black striped (non inductive?..)
I looked at Mills mra5, and ohmite audio gold ag5 or ag10(10w). Here is the thing:
Ag10 are 10w,and about 45mm long, so not sure I can squeeze them in. Besides, seems highercwarttage is ok, or am I wrong?
As far as the ag5, they are gone, can get either 0.20 ohm or 0.24 ohm. Seems 0.20 would generate more heat (watt=Volt*volt/resistance), and anyhow using something different than the original 0.22 ohm must have some impact, rebiasing the voltage drop maybe?
The basic question is will I get a noticeable improvement in sound? Is it worth it? Mills or audio gold? 0.20 or 0.24 ohm?