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Crown D-45 schematic


John Warren

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27 minutes ago, babadono said:

Thanks @shiva but I only need a few watts as these are going to be used to run the mid/tweeter driver on a pair of Jubes. Its just I am old school. If an amp is spec'd at 50 watts it should do it all day in my book. And on these old Crown units I thought Crown was truthful in their specs. I know on their current offerings the wattage specs are :pwk_bs:.

 I hear that,  an amp should not melt,  staying within its own specs. that is just a tad weak.   It is cool that you have Jubes going at home.  Enjoy.

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On 2/19/2018 at 2:06 PM, babadono said:

Thanks dude the @The Dude Is there a power supply page also?

So is the 2.7 ohm between DC rtn and chassis (R6) always in circuit? Or is there a jumper terminal on the back of the D75a just like the D75? Looks like pin 1 of XLR and sleeve of TRS at input jack is tied to DC rtn(signal ground, P16). Kind of hard to jibe this schematic with photos John Warren posted earlier.

Well I'm rambling, just thinking out loud, don't know if any body is following this. But I will continue with the mods to my non A units.

Sorry man, I don't know if I have a power supply page, I will dig around and see if I can find one.

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@The Dude Thanks it is mainly for @Cantilope and his D75a. My D75 non a types have followed exactly the schematic I posted earlier in this thread. Sure makes things a lot easier.  I finished the transformer replacement on the one I blew up:angry:. So now I am back to having two good modified units. I will not do full load tests for more than a couple minutes anymore:emotion-21:

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Well I got the third mod'd amp up and running into 4 ohm loads with the new zener circuit. But man o man the zeners and series dropping resistors are freaking HOT. The FLIR here at work just goes off scale >150 C. if I point it at the resistors. I'm afraid to button it up and call it done. I think I'll resort to making regulator boards for all. Now that I did the first one it only takes about an hour to duplicate.

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  • 2 weeks later...
  • 2 weeks later...
On 12/20/2016 at 4:12 PM, djk said:

"but has a lower slew rate than Mark's replacement. "

 

The SR doesn't need to be over 1V/µSec for the design. Q100, Q102 provide the voltage gain.

 

C101, C103 affect the audio quality way more than anything else in the circuit.

 

The stone-slow and anemic (gain wise) MJ15015 (a selected 2N3055) will be a problem driving low impedances.

@djk  A couple questions if you don't mind. Is there drop in replacement (T 03 case) for the MJ15015 that would improve this amp? And what about the circuit topology makes Q100 and Q102 provide all the voltage gain when there is an Opamp with 120dB of open loop gain driving them and included in the feedback loop?

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On 3/22/2018 at 11:21 AM, babadono said:

what about the circuit topology makes Q100 and Q102 provide all the voltage gain when there is an Opamp with 120dB of open loop gain driving them and included in the feedback loop?

 

At max input (~0.7VRMS) the op-amp serves as a differential input amplifier capable of ~2Vp-p ground referenced error signal.  Q102 is an emitter degenerated voltage-in, voltage-out amplifier with Q101 providing current limiting at clipping.  At max input, the signal at the base of Q102 is about 50mVp-p referenced to the -30V rail and, by way of the effective load on its collector, produces ~40Vp-p ground referenced signal at the bases of Q110 and Q187.    

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I understand the function of the differential to single ended converter( the first opamp after the input connector). What I don't fully understand is the second opamp. It is a typical non inverting amp and directly in the feedback loop with the rest of the discrete components and yet is provides almost none of the gain. I was surprised when I verified this by measurement. What is it they say....Live and learn.

Oh and you meant Q110 and Q107 not187 correct?

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6 hours ago, babadono said:

I understand the function of the differential to single ended converter( the first opamp after the input connector). What I don't fully understand is the second opamp. It is a typical non inverting amp and directly in the feedback loop with the rest of the discrete components and yet is provides almost none of the gain. I was surprised when I verified this by measurement. What is it they say....Live and learn.

Oh and you meant Q110 and Q107 not187 correct?

 

Yes, Q107, my copy of the schematic isn't clear.

 

I did explain the second op-amp.  It provides the error signal and that's its only job.  

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Feedback is sourced to the INV pin which has same phase relationship as non-INV signal from the gain pot.  The op-amp takes the difference (fractions of mV range) and amplifies the result (the error signal) and that's what drives the amplifier  The open loop gain of the op-amp is large and applied to the differences (both AC and DC).  

 

SPICE simulation shows the difference signal (blue) and the output (red) of the second op-amp (in this case the MC33079) at 20kHz sine wave, 0.5VRMS balanced signal sourced from 660 Ohm source impedance, gain and CMR pots at nominal.  The "glitches", when sent thru the amplifier, correct the signal to reduce distortion. 

 

The plot is taken a few milliseconds into the signal allowing caps to charge. 

 

 

error.jpg

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  • 1 month later...
  • 2 weeks later...
So I sent two D-75A's to John for "Updating".  
 
This is money well spent.  Turns a classic in a classic Gem.
 
The first thing noticed is no more harshness to the highs.  The high notes are clean and crashing symbols are no longer a reason to turn down the volume.
 
There is much more separation of instruments and detail.
 
The midbass has much more snap which I was able to pick-up on within a minute of listening.  I was not expecting this.  I can't comment on the lows and sub lows as I'm using an active crossover for those frequencies going to another amp.
 
Unlike the stock version, there is a little....,not a lot, of 3 dimension to the music.  Just enough to notice.
 
I will be sending him another pair in the near future.
 
jc
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