"So, what do you think ?"
Tons of DC off-set that will drift with temperature.
You will need to add a DC servo if you remove this cap.
Tons of DC off-set that will drift with temperature.
You will need to add a DC servo if you remove this cap.
DC feedback
"You will need to add a DC servo if you remove this cap"
Yes, but still the DC can flow though R25 and R26. So, indeed
the feedback of the DC signal will decrease but there will
be feed back.
Do you relly think that the aplifer will develop high DC output
that trimmpot P1 can not fix ?
Did you try it ?
Thanks for your response.
Guy
"You will need to add a DC servo if you remove this cap"
Yes, but still the DC can flow though R25 and R26. So, indeed
the feedback of the DC signal will decrease but there will
be feed back.
Do you relly think that the aplifer will develop high DC output
that trimmpot P1 can not fix ?
Did you try it ?
Thanks for your response.
Guy
"Yes, but still the DC can flow though R25 and R26. So, indeed
the feedback of the DC signal will decrease but there will
be feed back. "
When you remove C7,8 the amp becomes unity gain, the same as it is stock at DC. You would have to short out C7,8 to have normal gain. Now the DC off-set will be multiplied by the gain of R28, 26/25 (about X20).
"Do you relly think that the aplifer will develop high DC output
that trimmpot P1 can not fix ?"
I've had a problem with it when C7,8 were removed, that's why I mentioned it.
If 200mV of DC floating all around is OK, try it and see.
Even an opamp like the TL071 is only spec'd 10mV over temperature. With X20 gain this means it can float 200mV and be in spec.
Same problem with the Hafler.
An opamp with high DC accuracy and low drift normally does not sound good for audio, but can work OK for a DC servo.
the feedback of the DC signal will decrease but there will
be feed back. "
When you remove C7,8 the amp becomes unity gain, the same as it is stock at DC. You would have to short out C7,8 to have normal gain. Now the DC off-set will be multiplied by the gain of R28, 26/25 (about X20).
"Do you relly think that the aplifer will develop high DC output
that trimmpot P1 can not fix ?"
I've had a problem with it when C7,8 were removed, that's why I mentioned it.
If 200mV of DC floating all around is OK, try it and see.
Even an opamp like the TL071 is only spec'd 10mV over temperature. With X20 gain this means it can float 200mV and be in spec.
Same problem with the Hafler.
An opamp with high DC accuracy and low drift normally does not sound good for audio, but can work OK for a DC servo.
OK, thanks.
I will try it in the near future and I will let you know what are the results.
All the best,
Guy
I will try it in the near future and I will let you know what are the results.
All the best,
Guy
variac vs router speed control
In getting ready to replace the filter caps on my dh-200, I put out the word that I was looking for a Variac to borrow along with a brief description of what it does. A co-worker brought in a solid state speed controller for a woodworking router rated at 15 amps. It is my understanding that this unit works by varying the voltage, so it seems like it should provide the same effect as the variac, maybe just not as clean of a sine wave. Could I use this unit for reforming my capacitors (in the amp, fuses to the circuit boards removed) or shoud I continue my search for a true variac?
In getting ready to replace the filter caps on my dh-200, I put out the word that I was looking for a Variac to borrow along with a brief description of what it does. A co-worker brought in a solid state speed controller for a woodworking router rated at 15 amps. It is my understanding that this unit works by varying the voltage, so it seems like it should provide the same effect as the variac, maybe just not as clean of a sine wave. Could I use this unit for reforming my capacitors (in the amp, fuses to the circuit boards removed) or shoud I continue my search for a true variac?
I bought a used DH220 off of ebay several years ago. It's been a really nice amp and I enjoy the sound. However, I believe the caps are probably the originals and in need of replacement. I had planned to just buy Musical Concepts LC200s, but then I started reading this forum and got the crazy idea to totally start overhauling the amp! To that end I'm looking for some basic advice. I plan to do this in stages.
Stage 1 - replace the front end caps. Instead of the LC200s I was thinking about going with the BlackGate FK 10,000uF/80V or maybe even the 100Vs. A previous post by cdsherman also got me to thinking about replacing C1 and C5 (and maybe even C7 and C10) with a pair of BlackGate NX caps.
If I do this are there any other components I might need to replace in order to work with the BlackGate caps? If I do replace C1 and C5 with NX caps any suggestions on which ones?
Also, will I need to replace the diodes and if so any suggestions on that?
Stage 1 - replace the front end caps. Instead of the LC200s I was thinking about going with the BlackGate FK 10,000uF/80V or maybe even the 100Vs. A previous post by cdsherman also got me to thinking about replacing C1 and C5 (and maybe even C7 and C10) with a pair of BlackGate NX caps.
If I do this are there any other components I might need to replace in order to work with the BlackGate caps? If I do replace C1 and C5 with NX caps any suggestions on which ones?
Also, will I need to replace the diodes and if so any suggestions on that?
Hi
I just got a call tonight from my friend who has now one of my modified DH-200 amps and he says it is far way better (he only had good comments) than its Denon PMA-480R (which was an acclaimed amp at the time -1995). And the same friend with the same speakers did not like today's Cambridge amp compared to its Denon. So I conclude that the DH-200 (at least with the mods done) may still be a fine audio product even today after 25 years!
I just got a call tonight from my friend who has now one of my modified DH-200 amps and he says it is far way better (he only had good comments) than its Denon PMA-480R (which was an acclaimed amp at the time -1995). And the same friend with the same speakers did not like today's Cambridge amp compared to its Denon. So I conclude that the DH-200 (at least with the mods done) may still be a fine audio product even today after 25 years!
Modifying the DH-200
Rather than do this, you might want to consider a more modern circuit board created by the original DH-200 designer, Erno Borbely. This board (pair, actually) would include all needed caps, except the large mains filter caps. This is currently where my head is at.
Regards,
Rob
steveb7 said:
Rather than do this, you might want to consider a more modern circuit board created by the original DH-200 designer, Erno Borbely. This board (pair, actually) would include all needed caps, except the large mains filter caps. This is currently where my head is at.
Regards,
Rob
Rob,
Are you referring to a kit by Borbely? Which one? Will its PCB actually fit inside a DH-200/220 chassis? Tell us more.....
Are you referring to a kit by Borbely? Which one? Will its PCB actually fit inside a DH-200/220 chassis? Tell us more.....
weinstro said:
Rob,
Yes I know and this is what I have done too (but instead of using a high price Borbely kit I made my own design using some of the latest ideas from Erno since I am not sure that an easily Fit/form/function kit from Erno exists to be installed in the DH-200). But I agree that the Borbely kits are fantastic. Indeed, I have recently designed and built a jfet input mosfet output pre-amp based on a mix of Erno's circuits and it gave incredible resolution (from what I can hear) and the THD was the lowest I could measure using my home made THD metter (T.H.D. < 0.005%) if that matters of course.
My intention on my above post is simply to provide feedback on the fact that the stock DH-200 is a marvelous amp that probably still surpass mid-fi commercial products today and some cheaper tweaks making it close to the latest Borbely design make it sound even better and that compares to today's hi-fi amps (from what I have heard with my ears).
Dick West said:
According to Mr. Borbely, the DC50 will work, however the drive transistor needs to be moved to the heatsink. If you study his articles closely, it appears to be fairly easy to adjust the open loop gain for better bandwidth. It can also be converted to balanced input, but the Zin is very low in that case.
fab said:
Wow, that's great! However, the DC-50 driver board costs 120 Euros for a pair, and includes matched JFETs and film caps. I consider this to be a bargain. A bunch of film caps alone will probably run half that amount, shipped. And I don't have to go to the garage, build a rig, and then buy 20 JFETs to find some matches.
It's important to note that I'm planning on using just the driver board. i.e., reuse the Hitachi MOSFETs. If you add the outputs and heatsink, then it starts costing real money.
Cheers,
Rob
if you could still find "Servo 50" boards, they would work also. i think there was a US distributor for borbely that might have them.
mlloyd1
(a happy servo 50 user for years)
mlloyd1
(a happy servo 50 user for years)
Since my trusty old DH-200 needed to be taken out of service for repairs, I decided that this was a good opportunity to perform the POOGE modifications (rewiring, cap and resistor upgrades, etc.).While I’ve been doing this I have been referring to the DH-220 schematic (as well as the Audio Amateur POOGE mods) to incorporate changes made in the DH-220 where it makes sense (to me) to do so.
I have a question about some of the DH-220 capacitor value changes, which quite frankly, I don’t understand, and was looking for some guidance on what makes sense. In the DH-200 there are two capacitors, C13 between the gate and source of Q14 and C18, that connects between the drains of Q14/ Q15 and ground (the 2SK134 N-channel MOSFETs). C13 is 390pf and C18 is 680pf. The corresponding caps on the DH-220 are 680pf and .1mfd. respectively. Additionally, the DH-220 has a .005mfd cap (C408) hung from the drains of the P-Channel 2SJ49 MOSFETs to ground-the DH-200 lacks this capacitor. I would like to hear recommendations as to whether I should change the DH-200 to duplicate these changes in the DH-220. I assume that these capacitors are for stability; if anyone can explain to me how their values are supposed to be selected, I would also appreciate that.
Thanks.
I have a question about some of the DH-220 capacitor value changes, which quite frankly, I don’t understand, and was looking for some guidance on what makes sense. In the DH-200 there are two capacitors, C13 between the gate and source of Q14 and C18, that connects between the drains of Q14/ Q15 and ground (the 2SK134 N-channel MOSFETs). C13 is 390pf and C18 is 680pf. The corresponding caps on the DH-220 are 680pf and .1mfd. respectively. Additionally, the DH-220 has a .005mfd cap (C408) hung from the drains of the P-Channel 2SJ49 MOSFETs to ground-the DH-200 lacks this capacitor. I would like to hear recommendations as to whether I should change the DH-200 to duplicate these changes in the DH-220. I assume that these capacitors are for stability; if anyone can explain to me how their values are supposed to be selected, I would also appreciate that.
Thanks.
Mosfet Output caps
ilardi,
Regarding the mosfet caps, here is the answer from the DH-200/220 original designer:
My question was:
"..." I t was in the DH-200 original design. I thought that the purpose was to "equalize" the Cgs of N and P channels to get symetric waveform. I saw that in a lot of other designs too. "
Responses from Erno Borbely:
"The N-channel K134 MOSFETs should have 33pF caps between gate and drain, and NO caps between gate and source."
"Hi Fabien, yes, I started out with the g-s cap, but it turned out that it didn't solve the problem in all cases. The g-d cap did. I guess the other guys copied the way I did it"
ilardi,
Regarding the mosfet caps, here is the answer from the DH-200/220 original designer:
My question was:
"..." I t was in the DH-200 original design. I thought that the purpose was to "equalize" the Cgs of N and P channels to get symetric waveform. I saw that in a lot of other designs too. "
Responses from Erno Borbely:
"The N-channel K134 MOSFETs should have 33pF caps between gate and drain, and NO caps between gate and source."
"Hi Fabien, yes, I started out with the g-s cap, but it turned out that it didn't solve the problem in all cases. The g-d cap did. I guess the other guys copied the way I did it"
fab,
I'm a little confused by this. If I understand this correctly, the designer (Erno Borbely) is saying that both production designs are wrong since they both use g-s caps. Also, is the g-d cap (33pf) only to be used on the K134 and not also on the J49?
Also, was there any comment on the drain to ground cap that appears on both N and P channel MOSFETS in the DH-220?
Thanks for your help.
Terry Ilardi
I'm a little confused by this. If I understand this correctly, the designer (Erno Borbely) is saying that both production designs are wrong since they both use g-s caps. Also, is the g-d cap (33pf) only to be used on the K134 and not also on the J49?
Also, was there any comment on the drain to ground cap that appears on both N and P channel MOSFETS in the DH-220?
Thanks for your help.
Terry Ilardi
mosfet caps
Hi Terry,
You have perfectly understood. Only the K134 that is right. In fact if you check latest Erno's designs on the Borbely audio web site you will see that it is the case. And Erno confirmed that the g-d cap should have been used for the DH-200/220 and others too. However, the g-s cap is not that bad at all but as Erno has indicated the g-d cap improves stability even better.
I have not discussed this cap with Erno.
ilardi said:
Hi Terry,
You have perfectly understood. Only the K134 that is right. In fact if you check latest Erno's designs on the Borbely audio web site you will see that it is the case. And Erno confirmed that the g-d cap should have been used for the DH-200/220 and others too. However, the g-s cap is not that bad at all but as Erno has indicated the g-d cap improves stability even better.
ilardi said:
I have not discussed this cap with Erno.