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@CECAA850     @Rivernuggets

 

Since both of you had the Duke for a while, do either of you have a copy of Mark's notes about what he did in his mods to this amp? I know  I had them, thought it would be nice to keep a copy of his work with the amp. Mark had told me he would help with adjustments if I ever took this upon myself. I would be tempted one day to get one and try it.

 

BTW, it is rather incredible... dead silent, pushes the MWMs and LS tops with ease. Bass is very clean and tight from the MWMs.

 

Bruce

 

 

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Thanks so much! Just thought it would be nice to have the documentation. I know a scope was needed to finish the adjustments after the parts were replaced.

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  I do not know the Duke. But used interfaces with MC33079 quad opps.. Surface mount package. 

  Tried several replacements. None were an improvement. Most a step back.

    I would leave these alone. I doubt the TL084 is an upgrade. 

  Crown designed these with sound quality in as a priority. After all these years a recapping might pay dividends.

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5 hours ago, Panelhead said:

I would leave these alone. I doubt the TL084 is an upgrade. 

 

I'm not touching these at all. What Mark did with them is fine, exceptional, even. One of the things he did, which is superb, is modifying the input so it has a 47k input impedance. Perfect for me as my Merlin drives it just fine and I have no bass roll off as I would with  a modern low impedance input.

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It is a tube preamp that was designed and built by Mark Deneen aka Juicy Music Audio. Most tube preamps have a high output Z, and don't pair as well with SS gear with lower input Z, like 20kohms.

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On 9/20/2019 at 6:49 AM, Panelhead said:

  Crown designed these with sound quality in as a priority. After all these years a recapping might pay dividends.

 

The Crown D-45 used opamps with a fairly low slew rate. It helped to make them very stable while still sounding good. However, the lower slew rate made the top end softer, a little smeared. Mark upped the game @ 10 volts per microsecond. And replaced most of the power supply caps as well.

This required some fine tuning with an oscilloscope.

 

His history is he founded Paragon Audio in the '70s, sold it to ADC, retired aafter a bit, then started Juicy Music Audio as a one man cottage company. He is a smart guy. He also rubbed some fols the wrong way and is no longer on here. Miss a lot of the smart, curmudgeonly type we had on here. When you got past that, you could learn a lot.

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I posted in another thread when I was modifying my d75s. Basically the just of it was this. If you are using this amp unbalanced the only thing that effects the input impedance is the gain control potentiometer. All the other verbage about changing parts as if it was magic is non sense vis a vis the input impedance. Also if no changing of any of the compensation capacitors in the discrete amplifier sections of the amp was done then any increase in slew rate by just changing the error amp op amp is marginal. 

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I believe Mark did a lot more than swap out the chips. If you have an argument, you will have to find Mark and take it up with him.  🙄 I haven't heard an unmodified D45 next to this one, so I can't make a direct comparison. All I know is it is dead silent on my drivers, and it works  very well with my tube preamp.

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On 9/22/2019 at 12:45 PM, babadono said:

If you are using this amp unbalanced the only thing that effects the input impedance is the gain control potentiometer. All the other verbage about changing parts as if it was magic is non sense vis a vis the input impedance.

 

Did someone say it was magic? Just curious...  So, Crown says the input impedance w/unbalanced input is only 10k, which would quite rightly suck if I were using my tube preamp. Without finding the notes he sent me a long time ago to verify what he did, I'll just leave it closed up and stay happy with the way it works.

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Saw the thread title and said "wow Duke Spinner is back"🙂

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1 hour ago, Marvel said:

 

Did someone say it was magic? Just curious...  So, Crown says the input impedance w/unbalanced input is only 10k, which would quite rightly suck if I were using my tube preamp. Without finding the notes he sent me a long time ago to verify what he did, I'll just leave it closed up and stay happy with the way it works.

Sorry sometimes I get carried away. When someone claims they change a parameter in this case the input impedance and then offers no details on how that was done or the math or electrical engineering behind it except "I changed some parts, never mind which ones, the Great OZ has spoken", I call that magic or :pwk_bs:

So the unmodified input impedance was 10 K(unbalanced)? This is a D45 or D45A? On the D75s (non A) I modified the input impedance is 25k unbalanced and is solely determined by the gain potentiometer. The output from the pot goes into the input circuitry of the error amp. And the way Crown does their input strapping of the error amp there is no current being provided by the signal therefore the only impedance is the pot. I thought I checked the schematic of the A version and it was the same. I will check again.

Edit:

So there is a difference between the A and non A versions. In the older non A version when you use the 1/4 inch(unbalanced) input jack it bypasses the input diff amp and the signal goes directly to the gain pot. In the A version the signal always goes through the input diff amp and if you're using it unbalanced it is going to be 10K. So Mark probably changed the 8 resistors(4 per channel) around the input diff amp from 10k to 47k. That would make the unbalanced input impedance 47k. I wonder if he changed the feed forward compensation capacitor values to compensate for this almost 5X increase in resistance to keep the time constant of the input diff amp the same. I guess we'll never know.

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He was going to send me all the info and even have me call him if I needed help to make the adjustments. I can't remember if he did or not, but things kind of blew up concerning being on the forums. I had a lot in my work account which I may or may not have. I'll dig some more in my old emails and see if I can find what he did. 

 

It's all good, Eric. I never had any problem with him, but never followed him into left or right field, either.

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19 hours ago, Marvel said:

He was going to send me all the info and even have me call him if I needed help to make the adjustments. I can't remember if he did or not, but things kind of blew up concerning being on the forums. I had a lot in my work account which I may or may not have. I'll dig some more in my old emails and see if I can find what he did. 

 

It's all good, Eric. I never had any problem with him, but never followed him into left or right field, either.

I would love to see in more details exactly what Mark did. I am currently modifying the +/- 10 volt regulator boards that I built to make the amp stable with the opamps I chose when I modded my amps.  I am boosting them up to +/- 17 volts so there is no ability to overdrive the input diff amp with hot signals. I want to be able to have maximum levels from my mixer (and the Xilica active crossover) and be able to turn the gain down at the amps in the quest for the lowest noise floor.

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15 minutes ago, babadono said:

I would love to see in more details exactly what Mark did. I am currently modifying the +/- 10 volt regulator boards that I built to make the amp stable with the opamps I chose when I modded my amps.  I am boosting them up to +/- 17 volts so there is no ability to overdrive the input diff amp with hot signals. I want to be able to have maximum levels from my mixer and be able to turn the gain down at the amps in the quest for the lowest noise floor.

 

From what is mentioned in the thread that Carl saved, Mark changed the input on this one a sensitivity of 2.5 volts for full output instead of .775v. And then:

Part of their design goal (I read) was to prevent any possibility of RF runaway oscillations by strictly limiting slew rate. I get it - it makes
sense because they don't have the raw heatsinking capability on these little "chassis amps". But IMO, they went too far in slowing down
the amp. A lot of you guys (whomever that might mean) were suggesting that 10V/uS is probably a good figure for slew rate. I don't
disagree, except to say that might be the low end of a good range. The stock D45 comes in just under 4V/uS. I would like to get it to 10.
Here again, is a difference in "pro" versus "audio" thinking. In pro, reliability and ultimate stability caused a design compromise in treble
clarity (slower speed). For PA systems, losing a bit of treble speed is no big thing to trade off. For audio, it IS a big thing

 

 

I have emailed Mark. I will let you know if I hear anything back...

 

Bruce

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