Rod Elliot P.68 Phase?

[samutacaratti]

Hi all,

Having read numerous posts on the forum I have come to learn the importance of linear phase response and would like to take the audiophile route through my system.

I recently realised upon testing with a scope my cheapish subwoofer amplifier has very poor phase response deviating to almost 90 degrees at 120hz even if dc coupled. I found the capacitors which cause the problems but they are required for the amps stability and as the amp has had numerous faults and is running on virtually 0 bias current, I was looking into Rod's design substituting the MJ transistors with 2sc's.

I have emailed Rod but received no reply, could anyone advise from the design if it has a reasonably linear phase response with minimal deviation from 0 degrees (if anyone is able to deduce such information)?

Thanks

300/500W Subwoofer Power Amplifier

[Rafael L]

Originally Posted by samutacaratti

Hi all,

Having read numerous posts on the forum I have come to learn the importance of linear phase response and would like to take the audiophile route through my system.

I recently realised upon testing with a scope my cheapish subwoofer amplifier has very poor phase response deviating to almost 90 degrees at 120hz even if dc coupled.

Your subwoofer amplifier must have a crossover, the phase shift is normal.

[samutacaratti]

There is no crossover as I have stripped down the sub to the bare amp to use it with an PC based XO with a routed soundcard.

[richie00boy]

There must be a fault with the amp then. Any reasonable amp will have no phase shift to worry about in the audio band. The P68 will be fine and you can work out the phase shift points from the feedback network and input stage if it bothers you.

[samutacaratti]

Attached is a schematic of the stripped amp in use with the phase shift. The caps causing the big shift are circled in red. When removed the amp distorts heavily and is unstable.

Could anyone shine any light on this?

Thanks alot

[AndrewT]

Hi,
the input high pass filter 4u7F & {2k2+22k} is sufficiently low to allow all your bass and sub-bass signals to pass and yet still block DC and ultra low frequency.

The high pass built into the Negative FeedBack route cuts off a little higher. This should be lower than the input filter, change C4 to 180uF or 220uF.
Similarly C5 could usefully be a bit bigger, maybe 150uF.

[AndrewT]

what happened to ESP's P68?

Your new post shows the opamp with three caps directly connected to the -IN pin.
Many opamps become unstable with these in place. Maybe this amp has used these as compensation caps.

[samutacaratti]

Thanks for the advice with Rods design, from what people have said I gather Rods design will definitely have a linear phase response for atlest the lower frequency band (all "bass")?

My latest post was just to show the current manufactured design as Richie said it may be faulty - if the phase shift problem could be fixed I may put off building rods design for a while, ie if anyone can spot a flaw causing the phase shift in the original design that would be great I was resigned to having to totally make a new amp.

Thanks alot

[Rafael L]

How you measured the phase shift in your amp?

[wahab]

Increase C1 up to 68uF.
Increase C4 to 220 to 330uF.
C5 has a good value, increasing it bring no
improvement, quite the contrary.

Other than that, since it s a basses amp,
use a beefy power supply.