Hi DreadPirate,
You have too much in your quote there, do you want me to clean it up for you?
Anyway, I sold these in the 70's when there wasn't much good out. EV's cost a fortune and were excellent, Philips woofers came unglued easily (but were okay at lower levels). Marsland (Leigh Engineering, not McBride) where pretty good too. The Oaktrons were tough and good sounding. Cambridge were cheap knock offs of Marsland at the time. So yes, I slept very well. I sold all these except Cambridge (but tried samples) and EV. I always avocated a proper box construction using MDF or HDF (which rings by the way). I did feel bad when someone demanded Philips all the way through. Some of their 8's weren't too bad.
I've always had a problem with overly expensive wire. But let's not start a wire thread.
-Chris
You have too much in your quote there, do you want me to clean it up for you?
Anyway, I sold these in the 70's when there wasn't much good out. EV's cost a fortune and were excellent, Philips woofers came unglued easily (but were okay at lower levels). Marsland (Leigh Engineering, not McBride) where pretty good too. The Oaktrons were tough and good sounding. Cambridge were cheap knock offs of Marsland at the time. So yes, I slept very well. I sold all these except Cambridge (but tried samples) and EV. I always avocated a proper box construction using MDF or HDF (which rings by the way). I did feel bad when someone demanded Philips all the way through. Some of their 8's weren't too bad.
I've always had a problem with overly expensive wire. But let's not start a wire thread.
-Chris
How did that happen? Do what you need to do with my mispost, I don't think there is much constructive there anyway.
These don't look to be that great, but I once sold off some similiar looking pulls (actually those looked a bit better built) from some Criterion 100A speakers that apparently are highly sought after by knowledgeable speakerbuilders. These Oaktrons must have been highly thought of at the time, as they were mated with the Fisher 800-C which were "top of the line" according to the original purchaser.
I purchased an Adcom GFA-555 II from audioclassics.com yesterday, it will hopefully prove to be a good match with the Thiels.
Thanks for all the help and comments. You may not remember, but that Carver amp I asked about bridging was fixed with your advice several months back.
These don't look to be that great, but I once sold off some similiar looking pulls (actually those looked a bit better built) from some Criterion 100A speakers that apparently are highly sought after by knowledgeable speakerbuilders. These Oaktrons must have been highly thought of at the time, as they were mated with the Fisher 800-C which were "top of the line" according to the original purchaser.
I purchased an Adcom GFA-555 II from audioclassics.com yesterday, it will hopefully prove to be a good match with the Thiels.
Thanks for all the help and comments. You may not remember, but that Carver amp I asked about bridging was fixed with your advice several months back.
to answer your question about how bridging works, so you understand a little better why there are no specs for the carver bridged into 4 ohms.... and i suppose there aren't any specs for stereo into 2 ohms, either.......
let's say your power supply rails are +/- 65 volts (close enough) at 8 ohms, your peak current will be about +/- 8A into each speaker, at 4 ohms, it will be +/- 16A, and at 2 ohms +/- 32A. i'm going to guess here that carver used pairs of MJ15022/23 output devices (a very popular choice of output devices in the late '70's- early '80's). the fusing current on these devices is 15A, but since they're being used in pairs, that's 30A. but the devices can only sink that much current when the applied voltage across them is very low. you have the full 130 volts across them (65+65) when an amp is at a positive or negative peak, and as the voltage goes up, the current carrying ability of the transistor goes down. this is called Safe Operating Area (SOA). so long as the amp is operated at 4 or 8 ohms, the current and voltage of the transistors remain within the SOA. once you go to 2 ohms, you are operating the transistors outside the SOA, and can blow them up.
when you bridge an amp, you invert the phase of one channel, and put the outputs in series, so that when the R channel is at a positive peak, the L channel is at a negative peak, doubling the voltage across the load. since the output stages are wired in series, each one sees half the load. with an 8 ohm load, each output stage is driving 4 of the 8 ohms, as if you had two 4 ohm loads wired in series as well. when you drop to 4 ohms, it's as if you had two 2 ohm loads wired in series, and now the amp is back outside of the SOA again, and at the very least, overheating, at the worst....... letting out the "magic smoke"
let's say your power supply rails are +/- 65 volts (close enough) at 8 ohms, your peak current will be about +/- 8A into each speaker, at 4 ohms, it will be +/- 16A, and at 2 ohms +/- 32A. i'm going to guess here that carver used pairs of MJ15022/23 output devices (a very popular choice of output devices in the late '70's- early '80's). the fusing current on these devices is 15A, but since they're being used in pairs, that's 30A. but the devices can only sink that much current when the applied voltage across them is very low. you have the full 130 volts across them (65+65) when an amp is at a positive or negative peak, and as the voltage goes up, the current carrying ability of the transistor goes down. this is called Safe Operating Area (SOA). so long as the amp is operated at 4 or 8 ohms, the current and voltage of the transistors remain within the SOA. once you go to 2 ohms, you are operating the transistors outside the SOA, and can blow them up.
when you bridge an amp, you invert the phase of one channel, and put the outputs in series, so that when the R channel is at a positive peak, the L channel is at a negative peak, doubling the voltage across the load. since the output stages are wired in series, each one sees half the load. with an 8 ohm load, each output stage is driving 4 of the 8 ohms, as if you had two 4 ohm loads wired in series as well. when you drop to 4 ohms, it's as if you had two 2 ohm loads wired in series, and now the amp is back outside of the SOA again, and at the very least, overheating, at the worst....... letting out the "magic smoke"
Thanks for the bridging explanation, eventually it will make sense to me, at least I hope.
Re: your comment about 8A into 8 ohms, is that continuous? What about claims on some receivers, the GFA-555 for example, of 80A instantaneous current, I'm guessing that this is for a very small length of time, but I never see that stated. I also have a harmon kardon integrated the PM665 touting 60A, again, no other info or standard referenced. I don't think its pure marketing, these units have huge transformers and larger size caps than similarly rated (continous watts-wise) ones.
Re: your comment about 8A into 8 ohms, is that continuous? What about claims on some receivers, the GFA-555 for example, of 80A instantaneous current, I'm guessing that this is for a very small length of time, but I never see that stated. I also have a harmon kardon integrated the PM665 touting 60A, again, no other info or standard referenced. I don't think its pure marketing, these units have huge transformers and larger size caps than similarly rated (continous watts-wise) ones.
Hi unclejed613,
To add to the Carver case here. The supply voltages are switched up and down as required. So you may end up with a situation where the positive voltage is high and a guarantee of the negative rail going higher as well (more negative). I'd have to look at the schematic, but some early amps switched common supply rails. We now have probably 3/2 times the voltage across each output pair than there would be ordinarily. Not good for the planned dissipation and running these parts even further out of the SOA range.
Hi DreadPirate,
Peak current ratings should be taken with a grain of salt (big hunk). Sometimes this will be beyond the lead bond limit of the output device. No one said the amp survived the test!
-Chris
To add to the Carver case here. The supply voltages are switched up and down as required. So you may end up with a situation where the positive voltage is high and a guarantee of the negative rail going higher as well (more negative). I'd have to look at the schematic, but some early amps switched common supply rails. We now have probably 3/2 times the voltage across each output pair than there would be ordinarily. Not good for the planned dissipation and running these parts even further out of the SOA range.
Hi DreadPirate,
Peak current ratings should be taken with a grain of salt (big hunk). Sometimes this will be beyond the lead bond limit of the output device. No one said the amp survived the test!
At best!!
-Chris
when i was at Apt, i got asked by the boss to find out how much current an Apt-1 dumped into a dead short. so i built a "dead short" "brick wall" load, a 0.01 ohm 200w resistor made out of 0.5 ohm 5w ceramic resistors. during the short bursts while the speaker relay was connected running the amp at full input, i measured 50A peak. i ran the test for about an hour, the amp kept going click-click-click. i was surprised, because 25A per transistor (outputs were 2 MJ15022's, and '23's) is 10A beyond the bond limits of the transistors. the relay was controlled by SOA detectors. i wouldn't be surprised if that's where these manufacturers get their instantaneous current specs, from a similar test. btw, the "brick wall" lived up to it's name, it looked like one too...... i did make sure i put a brand new set of transistors in the amp after the test, as i didn't want to ship an amp to market that had had the transistors stressed like that.
you can do that test as long as your protection circuitry is adequate, if it isn't, be prepared for the consequences.......
you can do that test as long as your protection circuitry is adequate, if it isn't, be prepared for the consequences.......
i've got some sony's and such coming into the shop for open emitter resistors from mishandling of the speaker leads...... "i was hooking up my speakers, and now the protection relay trips whenever i go over 4 on the volume control"...... open emitter resistors causing dc offset.... so what made them fuss with speaker wires with the power on in the first place???????? "the wires only touched for a second..... 
" and it only takes a tenth of a second for a fusible resistor to go open.....
if somebody gets smart enough to read the instruction booklet, i'll be out of a job.............
" and it only takes a tenth of a second for a fusible resistor to go open.....if somebody gets smart enough to read the instruction booklet, i'll be out of a job.............
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