GK-2 cutoff frequency for SS section - Page 2 - diyAudio
Go Back   Home > Forums > Commercial Sector > Manufacturers > AKSA

AKSA Builders of market proven kits and modules at sound quality to rival the best in high end

Reply
 
Thread Tools Search this Thread
Old 30th January 2013, 08:40 PM   #11
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
Hi Jens,

Quick reply as I'm late for a appointment unfortunately.

Zout of the SS voltage stage in the GK2 is the reciprocal of the transconductance of the tube, so around 150R. So a 20K target impedance would be fine.

1uF is not enough, however. Impedance of a 1uF cap of 1uF at 10Hz is 16K, nearly the 20K of the source, so this would drop voltage by almost half, that is, 6dB.

I would increase this cap to 5uF, so that at 10Hz the impedance would be 3k2, so that all your very low frequency will give through. 5uF would also give you an impedance of 2k13 at 15Hz, MUCH lower than your 20K target and therefore about 0.5dB down, which is more than good enough bass.

Impedance for a cap is 160,000/uFxHz. Roughly, but get enough. You could use a good electrolytic here, since it has about 50V across it and with large polarising DC across it the sound quality will be outstanding.

Hope this helps,

Hugh

PS: My thanks to Osvaldo and Andy, good descriptions.
__________________
Aspen Amplifiers P/L (Australia)
www.aksaonline.com
  Reply With Quote
Old 30th January 2013, 09:42 PM   #12
andyr is offline andyr  Australia
diyAudio Member
 
Join Date: Nov 2003
Location: Melbourne (Oz, not Florida!)
Quote:
Originally Posted by Jens A. View Post
Hi Andy,

This whole thread is in fact about the "Sub Out" (= SS output)

The capacitors I am talking about are on the SS output, as recommended by Hugh, but they may need to be a somewhat higher value anyway.

Let's see what Hugh says!

Cheers,

Jens
Aah, I was confused by your use of "SS", Jens.

For Sub out, I use a pair of 4.7uF BG bipolar caps in what they call the "Super-E" configuration ... so 9.4uF, giving a -3dB frequency of 0.4Hz (with 43K Zin). So the phase error will run only up to 4Hz. However, those caps no longer seem to be available.


Regards,

Andy
  Reply With Quote
Old 31st January 2013, 08:06 AM   #13
Jens A. is offline Jens A.  Denmark
diyAudio Member
 
Join Date: Dec 2009
Location: Copenhagen
Quote:
Originally Posted by AKSA View Post
Hi Jens,

Quick reply as I'm late for a appointment unfortunately.

Zout of the SS voltage stage in the GK2 is the reciprocal of the transconductance of the tube, so around 150R. So a 20K target impedance would be fine.

1uF is not enough, however. Impedance of a 1uF cap of 1uF at 10Hz is 16K, nearly the 20K of the source, so this would drop voltage by almost half, that is, 6dB.

I would increase this cap to 5uF, so that at 10Hz the impedance would be 3k2, so that all your very low frequency will give through. 5uF would also give you an impedance of 2k13 at 15Hz, MUCH lower than your 20K target and therefore about 0.5dB down, which is more than good enough bass.

Impedance for a cap is 160,000/uFxHz. Roughly, but get enough. You could use a good electrolytic here, since it has about 50V across it and with large polarising DC across it the sound quality will be outstanding.

Hope this helps,

Hugh

PS: My thanks to Osvaldo and Andy, good descriptions.

Hi Hugh,

Thank you very much for this info.

Yes, 1 uF is obviously not enough.

Fortunately, I think I have a couple of Mundorf Silver/Oil 8.2 uF caps that I think will do very nicely for this job, so these will go in as soon as I have the time

I am certain that the Mundorfs will be very good in this position, and I might consider moving the two 1 uF teflon caps to the tube output instead - but I won't do this at the same time, as I want to make sure to be able to listen to each change in depth before I do the next one

Osvaldo and Andy: Thank you very much for your good input and patience!

Cheers,

Jens
__________________
"Life is too short for over-expensive hi-fi pretending to be high-end"
  Reply With Quote
Old 31st January 2013, 08:17 AM   #14
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
Jens,

My sincere pleasure!

With the advent of SMPSs in the last 20 years, there has been a steadily improvement in electrolytics, which are routinely now specced to 100KHz.

Further, in tube amps, we all find higher voltages, up to 140V in the GK2. When caps are polarised, even electros, with DC voltage, the sound quality comes a huge rise in subjective listening. Some very large film caps can introduced EMI/RFI because they have large areas, much more than electros, so I would invite you to have a few good quality electros (Sanyo Oscon, Nichicons, Panasonics) and compare them. You will be very surprised, particularly as you are working at 100Hz and less, where resolution and the best possible quality is not noticed by most listeners.

Cheers,

Hugh
__________________
Aspen Amplifiers P/L (Australia)
www.aksaonline.com
  Reply With Quote
Old 31st January 2013, 09:14 AM   #15
andyr is offline andyr  Australia
diyAudio Member
 
Join Date: Nov 2003
Location: Melbourne (Oz, not Florida!)
Quote:
Originally Posted by AKSA View Post

You could use a good electrolytic here, since it has about 50V across it and with large polarising DC across it, the sound quality will be outstanding.
Hi Hugh,

This raises an interesting issue, IMO ... where is the better place to put a coupling cap:

* on the output of the GK-2?

* or the input of the following component - in my case, an active XO?

Because you don't need them in both positions!

You say there is 50v across the output cap in the GK-2 ... which sounds great (as this large polarising voltage will certainly optimise the SQ of the output cap).

Whereas my active XO is running on +/-15v. So it would seem that (if I install the caps on the input of my active XO) there must be less polarising voltage ... so it would be much better to use them on the output of the GK-1?


Regards,

Andy
  Reply With Quote
Old 1st February 2013, 11:13 AM   #16
diyAudio Member
 
Osvaldo de Banfield's Avatar
 
Join Date: Dec 2011
Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
There is a fact that some people ignore or don't take into account until its too late and the circuits must be re-designed.

When using too big coupling caps together with a high impedance stage (output and input respectively), the cap will take more time to be the voltage across it stabilized when turning on and off the devices. So, in the very few instants, the audio is very distorted and/or noisier and or lower than in normal operation and POP's in the speakers. Also, the power supply must be severely filtered and or regulated so the ripple in the circuits at so low frequencies must be very low, and in case of unregulated ones, variations in line voltage will cause circuitry voltages an currents to vary, and this will be coupled to the audio chain causing woofer cones to move slowly at this frequencies causing non-linear distortion in them plus overheating in the coil because large DC offsets in it.

Good luck and greetings for all.
__________________
LW1DSE
  Reply With Quote
Old 1st February 2013, 08:18 PM   #17
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
Andy, Osvaldo,

The output from the bass cap on the GK2 is at the collector of a CFP, which has very low Zout, around around 80R. A 5uF cap here would be fine. The midrange and top end output of the GK2 is passed through a cathode follower.

The power supply is very carefully regulated, and the switch on is also controlled.

This request to slightly mod the GK2 is has been inquired by a couple of customers, but it is an after market change, something I normally would suggest as the bass quality of this preamp is carefully sorted in the original design. However, some always strive for purist, and yes, with minor change, this can be accommodated.

You cover the issues, thank you for your inside suggestion,

Cheers,

Hugh
__________________
Aspen Amplifiers P/L (Australia)
www.aksaonline.com

Last edited by AKSA; 1st February 2013 at 08:22 PM.
  Reply With Quote
Old 15th March 2013, 09:59 AM   #18
Jens A. is offline Jens A.  Denmark
diyAudio Member
 
Join Date: Dec 2009
Location: Copenhagen
Just a small update on the implementation of new caps for the SS (Sub Out) output of the GK-2.

I have now succesfully replaced the two 1 uF teflon caps with two 8.2 uF Mundorf Silver/Oil caps. I have no problems whatsoever with any kind of noise pickup, despite the fact that these caps are huge!

It should be mentioned that I have shielding walls in place inside the GK-2 enclosure (they have been there all along), and the caps are placed within these shielding walls.

Changing the cap value from 1 uF to 8.2 uF certainly improved bass extension, and although a bit rough when first fired up, the caps are now bedding in very nicely and becoming more and more tuneful.

While I had the GK-2 on the workbench, I also tried out another change, as I had a 10K Alps motorpot lying around. Hugh specifies 20K for the GK-2, but we thought that 10K would be fine instead of the original 50K Panasonic motor pot supplied with the GK-1R kit.

The 10K pot is fine, and works nicely. However, because my speakers are very efficient, regulation for low level listening is difficult.

I have now decided to go all out on the pot side and have ordered a TKD 2CP-2500MC motor pot. I have found a 25K version (at Michael Percy), which I think is fine in my case because of speaker efficiency, even though Hugh specifies 20K as standard.

The TKD is rated by many as the best motor pot available, and I have even heard of people saying that changing from the Alps to the TKD gave an improvement similar to a major equipment upgrade. Whether that will be true or not remains to be seen - I will forgo the hype and let my ears be the judge of that. At any rate, the TKD is the most precise motor pot around, so I will be looking forward to getting the precision and the right value into my GK-2(R)

Cheers,

Jens
__________________
"Life is too short for over-expensive hi-fi pretending to be high-end"
  Reply With Quote
Old 15th March 2013, 11:28 AM   #19
andyr is offline andyr  Australia
diyAudio Member
 
Join Date: Nov 2003
Location: Melbourne (Oz, not Florida!)
Quote:
Originally Posted by Jens A. View Post
Just a small update on the implementation of new caps for the SS (Sub Out) output of the GK-2.

I have now succesfully replaced the two 1 uF teflon caps with two 8.2 uF Mundorf Silver/Oil caps. I have no problems whatsoever with any kind of noise pickup, despite the fact that these caps are huge!

Changing the cap value from 1 uF to 8.2 uF certainly improved bass extension, and although a bit rough when first fired up, the caps are now bedding in very nicely and becoming more and more tuneful.

Cheers,

Jens
Thank you for your update - I am now inspired to put some larger caps on my "Sub Out".

However, I suspect that Mundorfs are an overkill for a signal output that doesn't really go above 100Hz? IOW, you could've used a different (ie. much cheaper) 8.2uF cap without ny sonic penalty in the bass region. Anyway, I will try something different.

Quote:
Originally Posted by Jens A. View Post

Hugh specifies 20K for the GK-2, but we thought that 10K would be fine instead of the original 50K Panasonic motor pot supplied with the GK-1R kit.
Mmmm, in the schematic Hugh sent me 10 years ago, he specified a 25K pot.

Quote:
Originally Posted by Jens A. View Post

The TKD is rated by many as the best motor pot available, and I have even heard of people saying that changing from the Alps to the TKD gave an improvement similar to a major equipment upgrade. Whether that will be true or not remains to be seen - I will forgo the hype and let my ears be the judge of that. At any rate, the TKD is the most precise motor pot around, so I will be looking forward to getting the precision and the right value into my GK-2(R)
When I replaced the stock motorised pot in my GK-1, I used a (manual) 25Kohm TKD stepped attenuator - as per Bob Crump's usage in his "Blowtorch" preamp. This is bracketed by some Vishay resistors - which Bob found improved the sound even further.

So I suggest if you want the convenience of remote volume ... you will be sacrificing some SQ.

Regards,

Andy
  Reply With Quote
Old 15th March 2013, 02:48 PM   #20
Jens A. is offline Jens A.  Denmark
diyAudio Member
 
Join Date: Dec 2009
Location: Copenhagen
Quote:
Originally Posted by andyr View Post
Thank you for your update - I am now inspired to put some larger caps on my "Sub Out".

However, I suspect that Mundorfs are an overkill for a signal output that doesn't really go above 100Hz? IOW, you could've used a different (ie. much cheaper) 8.2uF cap without ny sonic penalty in the bass region. Anyway, I will try something different.



Mmmm, in the schematic Hugh sent me 10 years ago, he specified a 25K pot.



When I replaced the stock motorised pot in my GK-1, I used a (manual) 25Kohm TKD stepped attenuator - as per Bob Crump's usage in his "Blowtorch" preamp. This is bracketed by some Vishay resistors - which Bob found improved the sound even further.

So I suggest if you want the convenience of remote volume ... you will be sacrificing some SQ.

Regards,

Andy
Hi Andy,

Yes, the Mundorfs are indeed overkill. I only chose them because I had them lying around idle. They may or may not make a difference in this position, but the thing is I know they are darn good, so after putting these in there I will have no doubts about this component any more. Hugh also suggested trying something else. I believe I would have chosen some Panasonics if it had been necessary for me to buy new ones

I believe the schematic with the 25K pot you are referring to is for the GK-1. However, the GK-1R was supplied with a 50K Panasonic motor pot, as you probably know. I have no idea how the 25K would perform in the GK-1, as I haven't tried it. The circuit and placement of the pot in the GK-2 are very different, but after the testing done with the 10K, I am sure that 25K is a good value in the GK-2.

Whether or not there is a sound quality penalty in using a pot compared to a stepped attenuator would probably depend on the attenuator and the pot. Some people claim that the TKD pots are better than some of the widely used attenuators like the DACT. I have no idea whether this could be true or not, but the TKD people certainly use some very sophisticated techniques in their pots ...

I also have no idea how the TKD pots compare to the TKD attenuators ...

Anyway, I do not want to loose my remote capabilities, so for my use, the best choice is the TKD motor pot

My pot was shipped from Michael Percy yesterday, so with a little luck and not too much customs hazzle, I should have it some time next week

Once installed, I will try to make an assessment if it makes a difference in sound quality. I am not expecting a lot, but even small improvements are nice!

Cheers,

Jens
__________________
"Life is too short for over-expensive hi-fi pretending to be high-end"
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Erse crossover cutoff frequency? rms1 Multi-Way 4 7th January 2013 03:42 AM
Does Potentiometer affect cutoff frequency clm811 Solid State 6 21st October 2011 10:27 AM
Frequency Cutoff habsrock93 Full Range 2 21st August 2007 01:32 PM
Crossover cutoff frequency ? Fradbut Multi-Way 3 22nd January 2007 09:31 AM
Reasonable low frequency cutoff markkanof Multi-Way 5 12th June 2002 10:14 PM


New To Site? Need Help?

All times are GMT. The time now is 04:30 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2