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PL 700 II REPLACEMENT MAIN VOLUME POTS
- Thread starter Greg
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Ok much research to do on this subject
first my take on impedance is the measurement “Impedance is for AC current as ohms is for DC current. Both are expressing resistance.
to add support George’s take on matching the pre to the Amp I copied this which helped me understand it a bit more
“If you think of your preamp as a source, and your power amp as a load, then you may consider that maximum power transfer can occur when the output impedance of the preamp matches the input impedance of the power amp. However, in high fidelity audio, it is typically considered optimum to have a source with low impedance connected to a load with high impedance. In that case, the power that can pass through the connection is limited by the higher impedance (so power transfer is not maximum), but the electrical voltage transfer is higher and less prone to corruption than if the impedances had been matched.
When matching preamps to power amps, a general rule of thumb is for the load (amp) input impedance to be at least 10 times higher than the source (preamp) output impedance to provide a suitably flat frequency response. Many prefer using a minimum ratio closer to 20 to 1, or having an amp with input impedance 20 times or more greater than the preamp output impedance.
With solid state preamps, this is generally not a problem since most have output impedance of only a few hundred ohms or less, while most SS amps have input impedance of at least 10K ohms. However, you must pay much closer attention when trying to match tubed preamps to SS amps, since many tubed preamps have an output impedance of several thousand ohms or greater. Another thing to watch is how the output impedance spec is reported, since it is sometimes limited to a measurement at a given frequency such as 1K Hz, while the actual output impedance may vary with frequency. It is not unusual for the output impedance of tubed preamps to rise significantly as the signal approaches a lower frequency of 20 Hz, because of the size of coupling capacitors used in the preamp. In these cases, a low frequency roll off can occur whereby, for the same power output, the lower frequencies drop in output compared to the rest of the frequency range, resulting in a loss of deep bass.”
Low output Impedance of the power amp influences speaker stability . This can be described as Dampening factor
A speaker resonates like a bongo drum, bong! How is the resonance controlled and damped? By the extremely low output impedance of a modern amplifier, the specification is its "damping factor".
Many good audio amplifiers have a damping factor of 400 , Phase Linear is even higher but I don’t think it is the 1000 they published, so the output impedance is 8 ohms/400= 0.02 ohms created by low impedance emitter-followers at the amplifier output and lots of negative feedback.
Then when the speaker tries to resonate it becomes a generator but the low impedance of the amplifier shorts its generation causing it not to move differently from the audio signal that is feeding it.
Try to turn a generator or push a speaker cone that is shorted. It is very difficult but they move or resonate easily when not shorted.
Many years ago, vacuum tubes were used in audio amplifiers and they had a high impedance. The output transformer was made to match the speaker impedance and the speakers were made to poorly damp themselves.
( sorry tube guys this probably not current with modern tube designs but does speak to matching tube components is critical)
first my take on impedance is the measurement “Impedance is for AC current as ohms is for DC current. Both are expressing resistance.
to add support George’s take on matching the pre to the Amp I copied this which helped me understand it a bit more
“If you think of your preamp as a source, and your power amp as a load, then you may consider that maximum power transfer can occur when the output impedance of the preamp matches the input impedance of the power amp. However, in high fidelity audio, it is typically considered optimum to have a source with low impedance connected to a load with high impedance. In that case, the power that can pass through the connection is limited by the higher impedance (so power transfer is not maximum), but the electrical voltage transfer is higher and less prone to corruption than if the impedances had been matched.
When matching preamps to power amps, a general rule of thumb is for the load (amp) input impedance to be at least 10 times higher than the source (preamp) output impedance to provide a suitably flat frequency response. Many prefer using a minimum ratio closer to 20 to 1, or having an amp with input impedance 20 times or more greater than the preamp output impedance.
With solid state preamps, this is generally not a problem since most have output impedance of only a few hundred ohms or less, while most SS amps have input impedance of at least 10K ohms. However, you must pay much closer attention when trying to match tubed preamps to SS amps, since many tubed preamps have an output impedance of several thousand ohms or greater. Another thing to watch is how the output impedance spec is reported, since it is sometimes limited to a measurement at a given frequency such as 1K Hz, while the actual output impedance may vary with frequency. It is not unusual for the output impedance of tubed preamps to rise significantly as the signal approaches a lower frequency of 20 Hz, because of the size of coupling capacitors used in the preamp. In these cases, a low frequency roll off can occur whereby, for the same power output, the lower frequencies drop in output compared to the rest of the frequency range, resulting in a loss of deep bass.”
Low output Impedance of the power amp influences speaker stability . This can be described as Dampening factor
A speaker resonates like a bongo drum, bong! How is the resonance controlled and damped? By the extremely low output impedance of a modern amplifier, the specification is its "damping factor".
Many good audio amplifiers have a damping factor of 400 , Phase Linear is even higher but I don’t think it is the 1000 they published, so the output impedance is 8 ohms/400= 0.02 ohms created by low impedance emitter-followers at the amplifier output and lots of negative feedback.
Then when the speaker tries to resonate it becomes a generator but the low impedance of the amplifier shorts its generation causing it not to move differently from the audio signal that is feeding it.
Try to turn a generator or push a speaker cone that is shorted. It is very difficult but they move or resonate easily when not shorted.
Many years ago, vacuum tubes were used in audio amplifiers and they had a high impedance. The output transformer was made to match the speaker impedance and the speakers were made to poorly damp themselves.
( sorry tube guys this probably not current with modern tube designs but does speak to matching tube components is critical)
Much to reread and think about today on this subject. Glenn has posted some good info. I do know I like the 50K pots in the 400's better than the 100K.
Have to wonder if it would be worthwhile to just eliminate them. I've never listened to a WOPL without them in circuit.
Joe, what would be gained in just eliminating the attenuators? Would be easy enough to just plug the hole nicely.
Have to wonder if it would be worthwhile to just eliminate them. I've never listened to a WOPL without them in circuit.
Joe, what would be gained in just eliminating the attenuators? Would be easy enough to just plug the hole nicely.
George you raised some good question such as if both 50k and 100k are turned to lowest resistance how could they be different in output or affect impedance. Now I have measured many pots and at minimum setting they vary and do show resistance and sometimes up to 100 ohms. New Bourne pots are good but they still have somimes 10-20 ohms on the average. I eliminate the high pass filter now and use the switch to other use the volume pots or bypass these pots on the 700’s. The 40 never had a high pass filter and with DCP protection not really needed on the amp input, as these amps were originally designed to be DC servo dive controls. Adding a bypass switch to the back of the 400 is easy although tight fit. A micro toggle is best.
now your statement about you liking the 50k over the 100k is that because of what? Do they sound different, does one put out more current (less resistance) or because one is better built. Just want to fully understand what you are saying.I also need to understand what Joe is saying about increasing impedance with the 50k and dropping impedance with the 100k and does that change over different pot positions ( effective resistance) as you turn them up ( lower resistance). That said if you move both pots have say 10k resistance is the effect on impedance the same or is it different for some reason I do not understand yet.
George I run all my amps at home with volume pots bypassed. That sounds best to me.
now your statement about you liking the 50k over the 100k is that because of what? Do they sound different, does one put out more current (less resistance) or because one is better built. Just want to fully understand what you are saying.I also need to understand what Joe is saying about increasing impedance with the 50k and dropping impedance with the 100k and does that change over different pot positions ( effective resistance) as you turn them up ( lower resistance). That said if you move both pots have say 10k resistance is the effect on impedance the same or is it different for some reason I do not understand yet.
George I run all my amps at home with volume pots bypassed. That sounds best to me.
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The 400 has no attenuators, just straight from the input RCA connectors to the input network of the control board. No attenuators is the best approach. Attenuators are for preamps, power amps should be straight through, or have an option to be straight through. Many top end power amplifiers have no attenuation controls, that job is left to the preamp.
The goal of preamplification is voltage gain, not current gain or power output. Power amplifiers do that job in addition to providing a modicum of voltage gain. Impedance matching between preamp and amp is irrelevant since you desire to pass the maximum voltage without any attenuation into the power amplifier to let the power amp do its job. The goal of a preamp is to have near zero output impedance and to provide signal path voltage gain.
The goal of preamplification is voltage gain, not current gain or power output. Power amplifiers do that job in addition to providing a modicum of voltage gain. Impedance matching between preamp and amp is irrelevant since you desire to pass the maximum voltage without any attenuation into the power amplifier to let the power amp do its job. The goal of a preamp is to have near zero output impedance and to provide signal path voltage gain.
George,
For the input attenuation to the PL700 amps you have to consider all the impedances such as the 220K Ohm cap bleed resistors on the input jacks, the 39K input resistors on the Control Boards, and of course the attenuation pots themselves. These are parallel resistances to the source.
With 50K attenuators, the preamp 'sees' 20K (actually 19,925.6 Ohms). With 100K attenuators, the preamp 'sees' 25K actually 24,883.99 Ohms).
Of course this is with the attenuators turned all the way down and calculated at DC, not the musical signal. But you see the difference is small.
For the input attenuation to the PL700 amps you have to consider all the impedances such as the 220K Ohm cap bleed resistors on the input jacks, the 39K input resistors on the Control Boards, and of course the attenuation pots themselves. These are parallel resistances to the source.
With 50K attenuators, the preamp 'sees' 20K (actually 19,925.6 Ohms). With 100K attenuators, the preamp 'sees' 25K actually 24,883.99 Ohms).
Of course this is with the attenuators turned all the way down and calculated at DC, not the musical signal. But you see the difference is small.
Glenn, reread Joe's post about the parallel resistance these pots put into the circuit.
Just now saw Joe's latest post, makes sense. I remember once he mentioned how these pots affect phase angle or something of the incoming signal.
The difference I hear from switching from 100K to 50K on the 400's was like they had higher output at lower volumes.
Now according to Mark's post, this is probably due to what the preamps is "seeing", a small change in DC resistance.
Overall very confusing to me for many reasons.
Joe's points about power amps makes sense, and Glenn's bypassing the pots also makes sense.
So, on the Series 2, why not ditch the pots and install a LED pilot light or plug in those holes? Thoughts?
Just now saw Joe's latest post, makes sense. I remember once he mentioned how these pots affect phase angle or something of the incoming signal.
The difference I hear from switching from 100K to 50K on the 400's was like they had higher output at lower volumes.
Now according to Mark's post, this is probably due to what the preamps is "seeing", a small change in DC resistance.
Overall very confusing to me for many reasons.
Joe's points about power amps makes sense, and Glenn's bypassing the pots also makes sense.
So, on the Series 2, why not ditch the pots and install a LED pilot light or plug in those holes? Thoughts?
Glen,
I just measured my Bourns pots at minimum DC resistance (maximum pass through) with my Fluke 77 just for fun. This is what I found:
50K Pots
3.9 Ohms, 4.5 Ohms, 4.8 Ohms, 4.9 Ohms, 5.1 Ohms, and 5.5 Ohms
100K Pots
3.5 Ohms, 3.7 Ohms, 3.7 Ohms, 3.8 Ohms, 3.8 Ohms, 3.9 Ohms, 3.9 Ohms, 4.2 Ohms, 4.3 Ohms, 4.4 Ohms, 4.4 Ohms, 4.4 Ohms, 4.4 Ohms, 4.6 Ohms, 4.7 Ohms, 4.9 Ohms, 5.1 Ohms, 5.3 Ohms, 5.6 Ohms, 5.8 Ohms.
The takeaways - more variation than I thought, but it was the same for 50K vs 100K. Bourns ships their pots fully CW (maximum pass through) are they trying to tell us something?!
Joe is right that the attenuation pots cause nothing but degradation to the sound. However, as an experimenter who constantly changes cables, sources, inputs, etc. I like to be able to attenuate the volume and bring it up slowly after I change something. I have plugged a "hot" input to a PL700B channel at full volume... The JBL Professional speakers took the 400 Watts in stride, but my ears did not.
For normal listening, the amplifier attenuators go fully CW and the musical power is controlled by the PL2000 volume control. I have experimented and found the sound is better with the amp full and the preamp low. With the amp low and the preamp turned up full, the sound is muddy and flat. A little.
I just measured my Bourns pots at minimum DC resistance (maximum pass through) with my Fluke 77 just for fun. This is what I found:
50K Pots
3.9 Ohms, 4.5 Ohms, 4.8 Ohms, 4.9 Ohms, 5.1 Ohms, and 5.5 Ohms
100K Pots
3.5 Ohms, 3.7 Ohms, 3.7 Ohms, 3.8 Ohms, 3.8 Ohms, 3.9 Ohms, 3.9 Ohms, 4.2 Ohms, 4.3 Ohms, 4.4 Ohms, 4.4 Ohms, 4.4 Ohms, 4.4 Ohms, 4.6 Ohms, 4.7 Ohms, 4.9 Ohms, 5.1 Ohms, 5.3 Ohms, 5.6 Ohms, 5.8 Ohms.
The takeaways - more variation than I thought, but it was the same for 50K vs 100K. Bourns ships their pots fully CW (maximum pass through) are they trying to tell us something?!
Joe is right that the attenuation pots cause nothing but degradation to the sound. However, as an experimenter who constantly changes cables, sources, inputs, etc. I like to be able to attenuate the volume and bring it up slowly after I change something. I have plugged a "hot" input to a PL700B channel at full volume... The JBL Professional speakers took the 400 Watts in stride, but my ears did not.
For normal listening, the amplifier attenuators go fully CW and the musical power is controlled by the PL2000 volume control. I have experimented and found the sound is better with the amp full and the preamp low. With the amp low and the preamp turned up full, the sound is muddy and flat. A little.
