I removed an active pre circuit for repair and left the remainder in so it could still be used. Shock, horror as it sounded very good indeed.
It's a bit different to a normal passive as the signal flow goes through several passives that were part of the active circuit after the pot.
CD Zout 100R > A20K pot > 2.2uF MKP cap > 68R in series with 100K to ground > power amp Zin 47K.
All the other passive pre's I've tried seemed to be a bit rolled off and lost some dynamics but doesn't seem to be the case here.
Is there benefits using passive components in conjunction with the pot in passive pre's or is this just an accident or an act of god?
It's a bit different to a normal passive as the signal flow goes through several passives that were part of the active circuit after the pot.
CD Zout 100R > A20K pot > 2.2uF MKP cap > 68R in series with 100K to ground > power amp Zin 47K.
All the other passive pre's I've tried seemed to be a bit rolled off and lost some dynamics but doesn't seem to be the case here.
Is there benefits using passive components in conjunction with the pot in passive pre's or is this just an accident or an act of god?
Hi,
passive pre-amp can give very good performance if all the ancilliaries are correctly proportioned to allow that performance to get out.
Since you didn't design it this way you have just been lucky to stumble into a well matched set-up.
Now that you know it can be done, there may be a little optimising left to improve things still further.
Start with cable capacitance and poweramp input filter components.
Give us all the info.
passive pre-amp can give very good performance if all the ancilliaries are correctly proportioned to allow that performance to get out.
Since you didn't design it this way you have just been lucky to stumble into a well matched set-up.
Now that you know it can be done, there may be a little optimising left to improve things still further.
Start with cable capacitance and poweramp input filter components.
Give us all the info.
rabbitz said:I removed an active pre circuit for repair and left the remainder in so it could still be used. Shock, horror as it sounded very good indeed.
It's a bit different to a normal passive as the signal flow goes through several passives that were part of the active circuit after the pot.
CD Zout 100R > A20K pot > 2.2uF MKP cap > 68R in series with 100K to ground > power amp Zin 47K.
All the other passive pre's I've tried seemed to be a bit rolled off and lost some dynamics but doesn't seem to be the case here.
Is there benefits using passive components in conjunction with the pot in passive pre's or is this just an accident or an act of god?
This circuit, if I've interpreted your description correctly, is basically just a high-pass filter, with a response that rolls off at about 20 dB per decade, below its cutoff frequency.
I plopped the circuit into LTspice and ran some AC Analysis (frequency and phase response) simulations.
I used an ideal voltage source as the CD, with 100 Ohms in series with it. I then have the 20K pot between signal and ground, with the wiper sending the signal onward to the 2.2uF cap, after which there's a 68 Ohm series resistor, 100K to ground, and then 47K to ground to represent the power amp's Zin. For the 2.2uF MKP cap, I used an ESR (Equivalent Series Resistance) of .005 Ohms.
Looking at the "output", just after the 68 Ohm resistor:
With the pot set for maximum volume (.001 Ohm/20K), the high-pass cutoff frequency, f(-3.1dB), is about 2.8 Hz, and there's about -0.1dB of signal at the power amp input. Phase shift at 10 Hz is about 12.7 degrees, and is about 1.3 deg at 100 Hz. After adding 0.5pF of parasitic capacitance to each resistor, and 3 nH parasitic inductance to the 2.2uF MKP capacitor, the cutoff frequency was about 2.26 Hz, and phase shift at 10 and 100 Hz were about 12.45 deg and 1.28 deg. The frequency response now begins to fall, after about 30 MHz, but has a small peak at around 1.8 GHz.
With the pot set to its midrange, 10K/10K, the cutoff frequency (f(-10.35dB)) is about 1.96 Hz, and there's about -7.35 dB of signal at the power amp's input. Phase shift at 10 Hz is about 11 degrees, and is about 1.12 deg at 100 Hz. After adding 0.5pF of parasitic capacitance to each resistor, and 3 nH parasitic inductance to the 2.2uF MKP capacitor, the cutoff frequency was about 1.95 Hz, and phase shift at 10 and 100 Hz were about 11 deg and 1.12 deg. The frequency response now begins to fall, after about 1 MHz, but levels out and then has a peak of -5.9 dB at around 1.8 GHz, before continuing to fall.
With the pot set for low volume, 19K/1K, the cutoff frequency (f(-29.33dB)) is about 2.2 Hz, and there's about -26.33 dB of signal at the power amp's input. Phase shift at 10 Hz is about 12.4 degrees, and is about 1.26 deg at 100 Hz. After adding 0.5pF of parasitic capacitance to each resistor, and 3 nH parasitic inductance to the 2.2uF MKP capacitor, the cutoff frequency was about 2.2 Hz, and phase shift at 10 and 100 Hz were about 12.4 deg and 1.26 deg. The frequency response now begins to rise, after about 1 MHz, but levels out and then has a peak of -6.75 dB at around 1.8 GHz, before falling again.
- Tom Gootee
http://www.fullnet.com/~tomg/index.html
Take out the cap
If your CD player is cap coupled, take out the 2.2 ufd capacitor. Also you can disconnect the 100K to ground. The 20K pot can be direct connected to the amplifier if desired.
A 10k log pot is a better match to an amplifier that is 47K. The 20K seems to be working okay for you.
If you have to leave in the 2.2 ufd capacitor, the 100K resistor to ground can still be disconnected.
Passive controls can work really well. But as said, the conditions have to be right. Some amplifiers are just not passive friendly. Same goes for sources.
George
If your CD player is cap coupled, take out the 2.2 ufd capacitor. Also you can disconnect the 100K to ground. The 20K pot can be direct connected to the amplifier if desired.
A 10k log pot is a better match to an amplifier that is 47K. The 20K seems to be working okay for you.
If you have to leave in the 2.2 ufd capacitor, the 100K resistor to ground can still be disconnected.
Passive controls can work really well. But as said, the conditions have to be right. Some amplifiers are just not passive friendly. Same goes for sources.
George
rabbitz said:I removed an active pre circuit for repair and left the remainder in so it could still be used. Shock, horror as it sounded very good indeed.
It's a bit different to a normal passive as the signal flow goes through several passives that were part of the active circuit after the pot.
CD Zout 100R > A20K pot > 2.2uF MKP cap > 68R in series with 100K to ground > power amp Zin 47K.
All the other passive pre's I've tried seemed to be a bit rolled off and lost some dynamics but doesn't seem to be the case here.
Is there benefits using passive components in conjunction with the pot in passive pre's or is this just an accident or an act of god?
With that setup your -3db @ 14hz, hardly what I would call good low frequency performance, besides this your just hearing what getting rid of unnecessary crap in the signal path sounds like, with an acceptable impedance match from cd to passive to amp.
Now get rid of the 2.2uf 68R and 100k and it should be even better.
Cheers George
It also depends on the characteristics of your inter-connects.
I built a passive pre-amp several months ago and found that it lost too much high and brightness. I ended up have to make it an active zero gain pre-amp. The conclusion is that you need an active amp or a circuit with low impedance to drive inter-connects with high capacitive load properly.
I built a passive pre-amp several months ago and found that it lost too much high and brightness. I ended up have to make it an active zero gain pre-amp. The conclusion is that you need an active amp or a circuit with low impedance to drive inter-connects with high capacitive load properly.
As expected, these things work best with short runs of the lowest capacitance and good dielectric cable you can get. I have a buffer amp in my preamp, with a switch to take it out of the circuit. The extra gain is rarely necessary, and with short low cap cables, it sounds just fine. This is one instance when cables do make an audible difference. It's also important that the input impedance of the power amp is flat over the audio range, but fortunately, most are.
We had a cable shootout at one of our audio club meetings between about 20 different interconnects. 40 odd members.
All were measured for capacitance, and the general concessus was that the lower capacitance < 100pf per foot cables sounded the best, and it so happened that generally they were also the most expensive ones, except for the odd one.
This was using an active pre with very low output impedance. And then a passive pre ( Lightspeed Attenuator), the cd source was 50 ohms output. Power amp in was 100K
The same result was noted with both pre's, active or passive that it's better to have low (<100pf per foot) interconnects.
Cheers George
All were measured for capacitance, and the general concessus was that the lower capacitance < 100pf per foot cables sounded the best, and it so happened that generally they were also the most expensive ones, except for the odd one.
This was using an active pre with very low output impedance. And then a passive pre ( Lightspeed Attenuator), the cd source was 50 ohms output. Power amp in was 100K
The same result was noted with both pre's, active or passive that it's better to have low (<100pf per foot) interconnects.
Cheers George
Thanks for your replies guys as I appreciate it.
Passive pre's are simple in concept but one of the hardest things to get right especially for the impedance challenged. I've read about the 100:1 rule on impedance matching but like most, I will not be able to achieve it.
The CD is a Sony CDP-X55ES which has a 100R resistor at the output. The pot is a 20K Alps remote RK27 series. The amp is a AKSA Lifeforce 55 and I've rechecked and the Zin is 42K and not 47K. The cables have a capacitance of 32pF/m so should work well.
I'll drag out the soldering iron this arvo and have a play and report back. I'll compare it to the SS pre that uses a 10K Alps black beauty in conjuction with the solid state section of an AKSA GK-1.
Passive pre's are simple in concept but one of the hardest things to get right especially for the impedance challenged. I've read about the 100:1 rule on impedance matching but like most, I will not be able to achieve it.
The CD is a Sony CDP-X55ES which has a 100R resistor at the output. The pot is a 20K Alps remote RK27 series. The amp is a AKSA Lifeforce 55 and I've rechecked and the Zin is 42K and not 47K. The cables have a capacitance of 32pF/m so should work well.
I'll drag out the soldering iron this arvo and have a play and report back. I'll compare it to the SS pre that uses a 10K Alps black beauty in conjuction with the solid state section of an AKSA GK-1.
AndrewT said:
I prefer to DC couple, so long as there are no nasties (dc offsets)present, and if there are, I'd rather work hard to get rid of the dc offset than use a cap.
But if you must have a coupling cap, then anything below -3db @ 3hz is fine.
But like I said I prefer dc coupled, as no matter how good the cap, no cap is better, a slight softening of leading edge to transients if it's a good cap, and just down right stodgy and veiled if it's a bad cap.
Cheers George
Re: Re: Re: Passive pre.... what's going on with this one?
Hi AndrewT
The path is like this:
CD 47uF, 100R
Cable 1m @ 32pF
Pot A20K
Cable 0.3m @ <10pF
Power amp 470nF, Zin 42K (I know the cap sounds low, but Hugh knows how to design these things)
Hi George
All the resistors and cap have been removed and yes, it does sound better. The clarity and soundstage has improved plus the dynamics have been retained which is a surprise for me as all my other passives tended to lose dynamics. The bass has lost a tad of punch and drive but the extension is there so it's not rolling off. These speakers have only had it's woofer changed from active + power amp to passive xo so there is still some tweaking to go and I can get the drive back. Why remove active you say? I want the amp to do all the range as it is that good and am willing to lose some punch but gain detail, quality and finesse.
So the passive has beat all the active pre's thus far except in a slight loss in bass drive but that's easy to fix by tuning the speaker to suit. The gain in all other areas make it worthwhile and this has been the first setup I've had that has resonded favourably to a passive bar a LM3875. I have yet to try a buffer (Zout <30R) between the CD and the pot and that will happen later in the week.
This process will go on for quite a while as fine tweaking and matching takes it's time when you change any component in the system.
I really appreciate your comments and ideas. Passive pre's, so simple yet so hard.
AndrewT said:
Hi AndrewT
The path is like this:
CD 47uF, 100R
Cable 1m @ 32pF
Pot A20K
Cable 0.3m @ <10pF
Power amp 470nF, Zin 42K (I know the cap sounds low, but Hugh knows how to design these things)
Hi George
All the resistors and cap have been removed and yes, it does sound better. The clarity and soundstage has improved plus the dynamics have been retained which is a surprise for me as all my other passives tended to lose dynamics. The bass has lost a tad of punch and drive but the extension is there so it's not rolling off. These speakers have only had it's woofer changed from active + power amp to passive xo so there is still some tweaking to go and I can get the drive back. Why remove active you say? I want the amp to do all the range as it is that good and am willing to lose some punch but gain detail, quality and finesse.
So the passive has beat all the active pre's thus far except in a slight loss in bass drive but that's easy to fix by tuning the speaker to suit. The gain in all other areas make it worthwhile and this has been the first setup I've had that has resonded favourably to a passive bar a LM3875. I have yet to try a buffer (Zout <30R) between the CD and the pot and that will happen later in the week.
This process will go on for quite a while as fine tweaking and matching takes it's time when you change any component in the system.
I really appreciate your comments and ideas. Passive pre's, so simple yet so hard.
This guy has some good info on cables- tweek info
Even though he doesn't like steel wire, I've been very happy with good old Belden 8421 coax. It's low capacitance and the foamed dielectric isn't too awful. I do believe dielectric is the critical item, not conductor material, so recommend going to a Teflon dielectric if you can afford it.
Even though he doesn't like steel wire, I've been very happy with good old Belden 8421 coax. It's low capacitance and the foamed dielectric isn't too awful. I do believe dielectric is the critical item, not conductor material, so recommend going to a Teflon dielectric if you can afford it.
Re: Re: Re: Re: Passive pre.... what's going on with this one?
Gootee,
can you run the numbers on this after we get confirmation of the RF filter?
and then substitute a 5kpot for the 20kpot.
I can see that the 470nF & 42k are cutting off the bass early. I wonder if the amp can respond to a change here?
I would like to see this filter dropped by two to three octaves, but the bottleneck might be inside.
Rabbitz,
the buffer might do more good after the pot rather than before, but that also depends on whether you will ever change those cable lengths to make volume adjustment more convenient or re-locate the power amp beside the speakers.
no RF filter in the power amp?rabbitz said:
Gootee,
can you run the numbers on this after we get confirmation of the RF filter?
and then substitute a 5kpot for the 20kpot.
I can see that the 470nF & 42k are cutting off the bass early. I wonder if the amp can respond to a change here?
I would like to see this filter dropped by two to three octaves, but the bottleneck might be inside.
Rabbitz,
the buffer might do more good after the pot rather than before, but that also depends on whether you will ever change those cable lengths to make volume adjustment more convenient or re-locate the power amp beside the speakers.
Conrad
This is the cable I'm using http://www.oyaide.com/e_audio/audio_products_files/pa-02.htm and I think the cable itself is made by Furukawa who are no slouch is this field. It's very similar in concept to Vampire CCC-II which is also a cable I'm fond of.
AndrewT
The amp certainly does not have a problem in the bass area with -1dB points at 15Hz and 85KHz and the bass is one of the strong points in the reviews and I have to agree. It's just a matter of seeing if it's a good candidate for a passive pre, and from what I've heard in the last 24 hours, I'd have to say yes. A 10K pot would be better I think but the 20K is doing well.
The circuit information however is proprietary and belongs to Hugh Dean so there is no information on the RF filter and I respect that. This is the amp I'm referring to. http://www.aksaonline.com/products_2_3.html
I'd say the cable lengths would remain constant due to the configuration of the room and rack and if the power amps were ever moved near the speakers, then it would have to be active pre for sure. You're right and read my mind as I was going to try the buffer before and after the pre.
A slight bit of tuning of the speaker port and maybe some crossover tweaks will return the bass drive to where it was with an active pre (we're not talking much here as it's only just noticeable) which is not half bad for a passive pre..... I'm impressed. This power amp has responded extremely well and has given better results than the heaps of others I've tried with a passive. It's easier to tweak a speaker to suit an amp than the other way around.
Thanks for all your advice.
This is the cable I'm using http://www.oyaide.com/e_audio/audio_products_files/pa-02.htm and I think the cable itself is made by Furukawa who are no slouch is this field. It's very similar in concept to Vampire CCC-II which is also a cable I'm fond of.
AndrewT
The amp certainly does not have a problem in the bass area with -1dB points at 15Hz and 85KHz and the bass is one of the strong points in the reviews and I have to agree. It's just a matter of seeing if it's a good candidate for a passive pre, and from what I've heard in the last 24 hours, I'd have to say yes. A 10K pot would be better I think but the 20K is doing well.
The circuit information however is proprietary and belongs to Hugh Dean so there is no information on the RF filter and I respect that. This is the amp I'm referring to. http://www.aksaonline.com/products_2_3.html
I'd say the cable lengths would remain constant due to the configuration of the room and rack and if the power amps were ever moved near the speakers, then it would have to be active pre for sure. You're right and read my mind as I was going to try the buffer before and after the pre.
A slight bit of tuning of the speaker port and maybe some crossover tweaks will return the bass drive to where it was with an active pre (we're not talking much here as it's only just noticeable) which is not half bad for a passive pre..... I'm impressed. This power amp has responded extremely well and has given better results than the heaps of others I've tried with a passive. It's easier to tweak a speaker to suit an amp than the other way around.
Thanks for all your advice.
rabbitz said:
Rabbitz, your right in assuming by dropping the passive pot from 20k to 10k you may see an improvement in bass attack, you are well known in the Newcastle area, I have sent a few of my Lightspeed Attenuators up there maybe you could ask around to borrow one, they are the equivalent of 7k-10k constant input and output and will sound even better than any passive pot.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=80194
Cheers George
George
I've been following the Lightspeed thread since the start and very intrigued.... been thinking about getting one for quite a while. Been working out how it goes in my existing pre case, remote volume etc.
I still have quite a bit of testing to do with a few different variations. I can always drop the value of the pot by resistors from the input to the wiper but easier to try another pre I have with a 10K Alps black beauty.... I'll disable the active circuit.
Most of the bass variation has been sorted and was caused by the removal of the plate amps when the new amp took over. Extra volume in the speakers which in turn pushed the Fb down so was just a matter of reducing the port length.... easy.
Well known in Newcastle... hope it's not the law enforcement agencies.
I've been following the Lightspeed thread since the start and very intrigued.... been thinking about getting one for quite a while. Been working out how it goes in my existing pre case, remote volume etc.
I still have quite a bit of testing to do with a few different variations. I can always drop the value of the pot by resistors from the input to the wiper but easier to try another pre I have with a 10K Alps black beauty.... I'll disable the active circuit.
Most of the bass variation has been sorted and was caused by the removal of the plate amps when the new amp took over. Extra volume in the speakers which in turn pushed the Fb down so was just a matter of reducing the port length.... easy.
Well known in Newcastle... hope it's not the law enforcement agencies.
Re: Re: Re: Re: Re: Passive pre.... what's going on with this one?
Hi Andrew,
No problem, except that I'm not sure about what the configuration is, exactly.
Would it be: Source, then a series 47uF, series 100R, 32pF to gnd, 20k (or 5K) pot, wiper out, to 10pF to gnd, series 470nF, 42K to gnd ?
And if there was an RF filter, I assume that it would go either before or after the 470nF (and might consist of a few hundred series ohms and a few hundred pF to gnd, and then maybe another few hundred series ohms).
Correct?
OT: I saw the "The Last King of Scotland", recently. Interesting movie, in some ways. It's about Idi Amin's young Scottish doctor. (Apparently, Amin loved "all things Scottish". I never would have guessed THAT.)
- Tom Gootee
http://www.fullnet.com/~tomg/index.html
AndrewT said:
Hi Andrew,
No problem, except that I'm not sure about what the configuration is, exactly.
Would it be: Source, then a series 47uF, series 100R, 32pF to gnd, 20k (or 5K) pot, wiper out, to 10pF to gnd, series 470nF, 42K to gnd ?
And if there was an RF filter, I assume that it would go either before or after the 470nF (and might consist of a few hundred series ohms and a few hundred pF to gnd, and then maybe another few hundred series ohms).
Correct?
OT: I saw the "The Last King of Scotland", recently. Interesting movie, in some ways. It's about Idi Amin's young Scottish doctor. (Apparently, Amin loved "all things Scottish". I never would have guessed THAT.)
- Tom Gootee
http://www.fullnet.com/~tomg/index.html
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