Has anyone tested, or have an idea, of the output current bottleneck in the 303?
The usual suspects are:
1- transformer voltage collapse
2- power supply regulator circuit collapse/limiter
3- amp output stage (or drive) limiter.
I figure the recommended 67V supply voltage is a function of how much the pre-regulator 80V collapses under high current demand.
So may be a higher capacity transformer would also allow a little higher regulator voltagee.
I would like my 303 to breathe better with my sub-6Ohms low-impedance speakers.
The usual suspects are:
1- transformer voltage collapse
2- power supply regulator circuit collapse/limiter
3- amp output stage (or drive) limiter.
I figure the recommended 67V supply voltage is a function of how much the pre-regulator 80V collapses under high current demand.
So may be a higher capacity transformer would also allow a little higher regulator voltagee.
I would like my 303 to breathe better with my sub-6Ohms low-impedance speakers.
I would first verify whether the circuit is current or voltage limited.
If current limited (which I suspect), upping the voltage increases chance of failure.
Do you have a schematic to post?
Jan
If current limited (which I suspect), upping the voltage increases chance of failure.
Do you have a schematic to post?
Jan
The first limit you will hit is the current limiting. With modern output transistors (MJ15003) you can raise it by using two or even three current limiting diodes in series for each side.
My first goal is not to increase the voltage, just to have more current reserve.
If I play around with the voltage it'll be months after the first mods.
And yes, I will do the MJ15003 upgrade before.
If I play around with the voltage it'll be months after the first mods.
And yes, I will do the MJ15003 upgrade before.
Sorry, I replied to the wrong post, meaning to reply to jan.didden. My deepest, sincere apologies.
Upgrading the output transistors and raising the current limit will burden the PSU though. You may have to reduce the voltage to allow greater ripple on the input supply (80V or so). I don't recall the PSU having a current limit, only the amplifier. But you may have to raise the driver current in the PSU. I'd probably put an MJ21194 in the PSU instead of the - was it a 40411 equivalent - as that may have enough gain at a higher current.
I would be concerned about the transformer, filter capacitors current handling though .....
I would be concerned about the transformer, filter capacitors current handling though .....
The 303 has a basic current limit circuit with no SOAR complications.
This means that the limit was set cautiously at about 2A.
My feeling is that you best bet is simply to look out for a 306, which is perfectly happy with low impedance speakers and has most of the 303 sound
This means that the limit was set cautiously at about 2A.
My feeling is that you best bet is simply to look out for a 306, which is perfectly happy with low impedance speakers and has most of the 303 sound
Higher than 2A. The 303 was rated at 50W into 8 ohms, so must be able to provide at least 3.6A, which I think you will find is the case.
Yes you are correct, the diodes start conducting at about 3.5A
Having 0.3R output stage resistors is not very efficient with a ~ 4 Ohm loudspeaker either
Having 0.3R output stage resistors is not very efficient with a ~ 4 Ohm loudspeaker either
For 4 ohms an output power of only 25W would be right.
If the op wants a low impedance version, it may be better to swap the PSU for a lower voltage higher current option, and actually set the current limit higher as suggested. Probably, a replacement transformer would fit the space available, but all the drive current capability in the PSU and amplifier would need to be checked.
Modern capacitors generally are smaller than older electro's so it may be possible to put in larger capacitors in the same space, to maintain the output low frequency point.
Not a task to be undertaken lightly, but possible, I would think. The main point being to determine what needs to change, then seeing whether it can be accomplished in the same tin as the original, though that would no longer be possible to call a "303".
If the op wants a low impedance version, it may be better to swap the PSU for a lower voltage higher current option, and actually set the current limit higher as suggested. Probably, a replacement transformer would fit the space available, but all the drive current capability in the PSU and amplifier would need to be checked.
Modern capacitors generally are smaller than older electro's so it may be possible to put in larger capacitors in the same space, to maintain the output low frequency point.
Not a task to be undertaken lightly, but possible, I would think. The main point being to determine what needs to change, then seeing whether it can be accomplished in the same tin as the original, though that would no longer be possible to call a "303".
Presumably you have genuine reasons for this ?
IMO the 303 wasn't a very good amp.. despite Fanboys.
IMO the 303 wasn't a very good amp.. despite Fanboys.
The 67 volt should be a constant as the regulator is a stabilised series pass type. Be aware the the it is the 'ground' of the single rail supply that is regulated relative to the reservoir cap negative terminal and so you have to make sure you measure the 67 volts from the correct point.
I agree. It can be improved with epibase 2N3055's and a few tweaks. I've doubled the frequency response in a 303 "clone" and it sounds better. Not investigated the reasons because even better performance possible with fully complementary designs these days.