Last word on VHT "Zobel Network" ???
Greetings Gents.
I wish to thank one and all who have replied to my post. I have learned a lot on this one ! It makes me feel better, reading the last few posts. In reply to teemuk's wondering if the possibility of the texture switch/Zobel effect could be broken, the answer is YES. I actually sent the first of 2 Special 6 Ultras back to Musician's Friend because I was convinced that the texture switch was defective. When I got the second amp, and the texture switch effect was still not audible to me, I brought the issue to this forum, as well as emailing VHT about it. They insist that it is a subtle effect, and is only really effective when the amp is pushed really hard. However, as I have stated before, I can't tell any effect with the amp maxed both on the clean channel, and on the Ultra channel with the push-pull boost engaged, plus the footswitchable volume/tone bypass boost engaged as well.
Thanks to Malcolm Irving on the value change of R1 to 10kOhm. If changing the values of R43, R42, and R49 per VHT's advice will lower D.C. supply voltages in the pre-amp, will this adversely affect output power, or be a disadvantage tonally ?
In reply to Nigel Goodwin, I think that you have put into a good technical argument, exactly what I have felt about this "Zobel Network" almost from the start. I believe that this circuit(texture switch) is the one thing about this amp that is badly done. In this case, a Zobel Network does not seem to be well suited for it's intended use. As you put it Nigel, the capacitance values are not even typical for a Zobel Network. This leaves ME thinking,....."What were THEY thinking". I was considering doubling the values of "C" in this circuit, just to get something out of it. However, the amp also has a standard tone control, which does a fine job of rolling off treble(If that's what one wants). The one thing that bothers me the most, is that VHT claims that they have never had a problem or complaint about the texture switch before. In my case, I have to go with my ears, and call a lemon a lemon !
Thanks again guys, for ALL of the good info I received in your replies. I truly appreciate every comment. I'll be thinking on what to do with the texture switch for a while, so if anybody has a good idea for a circuit I could hook up to the 3-way switch, do let me know. The amp looks better with it intact, and it might as well DO something as well ! If nothing else comes up, I'll double the values of "C".
Best Regards to all,...................tonequester.
Greetings Gents.
I wish to thank one and all who have replied to my post. I have learned a lot on this one ! It makes me feel better, reading the last few posts. In reply to teemuk's wondering if the possibility of the texture switch/Zobel effect could be broken, the answer is YES. I actually sent the first of 2 Special 6 Ultras back to Musician's Friend because I was convinced that the texture switch was defective. When I got the second amp, and the texture switch effect was still not audible to me, I brought the issue to this forum, as well as emailing VHT about it. They insist that it is a subtle effect, and is only really effective when the amp is pushed really hard. However, as I have stated before, I can't tell any effect with the amp maxed both on the clean channel, and on the Ultra channel with the push-pull boost engaged, plus the footswitchable volume/tone bypass boost engaged as well.
Thanks to Malcolm Irving on the value change of R1 to 10kOhm. If changing the values of R43, R42, and R49 per VHT's advice will lower D.C. supply voltages in the pre-amp, will this adversely affect output power, or be a disadvantage tonally ?
In reply to Nigel Goodwin, I think that you have put into a good technical argument, exactly what I have felt about this "Zobel Network" almost from the start. I believe that this circuit(texture switch) is the one thing about this amp that is badly done. In this case, a Zobel Network does not seem to be well suited for it's intended use. As you put it Nigel, the capacitance values are not even typical for a Zobel Network. This leaves ME thinking,....."What were THEY thinking". I was considering doubling the values of "C" in this circuit, just to get something out of it. However, the amp also has a standard tone control, which does a fine job of rolling off treble(If that's what one wants). The one thing that bothers me the most, is that VHT claims that they have never had a problem or complaint about the texture switch before. In my case, I have to go with my ears, and call a lemon a lemon !
Thanks again guys, for ALL of the good info I received in your replies. I truly appreciate every comment. I'll be thinking on what to do with the texture switch for a while, so if anybody has a good idea for a circuit I could hook up to the 3-way switch, do let me know. The amp looks better with it intact, and it might as well DO something as well ! If nothing else comes up, I'll double the values of "C".
Best Regards to all,...................tonequester.
Yes, they are more than decade higher.
Well, the generic Zobel network is supposed to compensate the inductive characteristics of a loudspeaker load; the impedance of the load increases towards higher frequencies. Connecting a properly chosen capacitor in parallel to inductance levels out the impedance. The generic Zobel is also supposed to do that compensating ABOVE audible bandwidth.
Increasing the capacitance by more than a decade shifts the operating point of the filter lower in frequency, that is to to audible bandwidth. The whole purpose of the circuit changes: The filter, which formerly was inaudible and mainly there for overall stability, now becomes a tone control. That is what they were thinking.
It rolls of treble BEFORE the power amp. If the power amp clips (which usually is the intention with these low power amp thingies) the higher order harmonics from distortion will, however, seemingly increase treble response, "brightness of tone", once again.
If you want to affect the distorted tone then the most effective place to do that is actually after the distorting stage.
All kind of "EQ'ing" will have very different effects when it happens before distortion and when it happens after distortion.
Last edited:
More on the mysterious Zobel.
Greetings, and thanks teemuk.
I had not thought about the placement, circuit wise, of the tone control. I appreciate you bringing that up. However, I should make it clear, that VHT never makes any mention of the Zobel network(Texture switch) being something to help prevent instability of the amp, or that it actually works above the audible frequencies. What they do talk about, and I quote their own manual , " Texture switch : This 3-position rocker switch selects the output's high- frequency roll off point". Kind of sounds like the way I'd describe a tone control. They don't mention Zobel or Boucherot, nor do they mention anything about stability. I got all of that from my emailing them about the inaudible "quality" of the switch. They also never mention that one must overdrive the amp considerably and fool around with all of the other controls to get results. This, they also told me by email. Again, thanks for reminding me of the placement of the tone control. Perhaps this fact, will work out well with my plan to double the value of the Zobel caps. Maybe there would be some interaction between the tone control and the Zobel, instead of just having additive effects.
Thanks again teemuk, and all of the other guys that have posted replies. I appreciate your time and effort, and the fact that you are sharing generously with one who knows that he is beneath you in talent. Best Regards,....tonequester.
Greetings, and thanks teemuk.
I had not thought about the placement, circuit wise, of the tone control. I appreciate you bringing that up. However, I should make it clear, that VHT never makes any mention of the Zobel network(Texture switch) being something to help prevent instability of the amp, or that it actually works above the audible frequencies. What they do talk about, and I quote their own manual , " Texture switch : This 3-position rocker switch selects the output's high- frequency roll off point". Kind of sounds like the way I'd describe a tone control. They don't mention Zobel or Boucherot, nor do they mention anything about stability. I got all of that from my emailing them about the inaudible "quality" of the switch. They also never mention that one must overdrive the amp considerably and fool around with all of the other controls to get results. This, they also told me by email. Again, thanks for reminding me of the placement of the tone control. Perhaps this fact, will work out well with my plan to double the value of the Zobel caps. Maybe there would be some interaction between the tone control and the Zobel, instead of just having additive effects.
Thanks again teemuk, and all of the other guys that have posted replies. I appreciate your time and effort, and the fact that you are sharing generously with one who knows that he is beneath you in talent. Best Regards,....tonequester.
- Status
- Not open for further replies.