Removing Balance & Panorama from Crown IC 150a Preamp

mitchcurl

Member
2016-06-14 1:19 am
Just happened upon a vintage Crown IC 150A pre in my friends basement. Thing sounds great, but I couldn't stand having a balance pot in the signal flow so I cut the thing out hoping to simply soldier the pot leads to completely bypass the pot. Stupid idea I'm now realizing. I have only managed to get a fraction of the signal to return and only out of one headphone. I did this by fiddling around in the amp until I found a connection that produced some signal. Anyone have any ideas how to make this bypass actually work?
 

mitchcurl

Member
2016-06-14 1:19 am
Thanks for the help rayma! I have 2 questions about this:
1. The model that I have does not label the PCB. Do you know where I can find a schematic to locate this cap? (I have looked all over the place with little luck)
2. Just to double check: you want me to solder to the wiper of the VOLUME pot, not the BALANCE?

sorry im quite a noob...
 
Crown didn't use a very good opamp so replacing that with something better would be a good idea. The RIAA preamp looks like a knock off of a James Bongiorno design which you can see in the June 1972 issue of Popular Electronics. Crown did add a cascode first stage and change the design to work off +/- 18 volt power supplies so it's not a total copy.
Here's a link to the Bongiorno preamp article. http://www.americanradiohistory.com/Archive-Poptronics/70s/1972/Pop-1972-06.pdf
 

rayma

Member
2011-04-29 8:37 pm
The RIAA preamp looks like a knock off of a James Bongiorno design which you can see in the June 1972 issue of Popular Electronics. Crown did add a cascode first stage and change the design to work off +/- 18 volt power supplies so it's not a total copy.

Seems that isn't correct. Crown IC 150 Review
Crown IC 150 Stereophonic Preamplifier (1971-1977)
He might have consulted for Crown, though.
 
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llwhtt

Member
2008-06-12 3:43 am
SoCal
I'm a fan and collector of all things Bongiorno and have never seen that article. This is the first time I've seen in print a relationship between JB and Dan Meyer of SWTPC. There was some controversy over who came up with the dual differential input stage used in the SAE and GAS amplifiers by JB and the Tigersaurus of Dan Meyer. Maybe we'll never know!

Now back to the Crown IC-150.

Craig
 
Thank you guys for taking the time to help me out with this! I have managed to find the capacitor c28 on the schematic (its name only appears on one schematic and it has only a "1" as the value...) I have tried to find it by locating the 2M resistor that it connects to on the main board and what I believe to be the opamp section with no luck... Here are some pictures. crown cap28.JPG

IMG_1275.jpg

if anyone has any hints or ideas, I would love to hear them.
 
The IC 150 isn't a very good early solid state design, but it is a great chassis.
Rather than try to simplify the signal path , you'd be better off to build a Pass diy or other modern design in the chassis , get new jacks and volume control.

raspberries.

The IC150 benefits from some a simple power supply mod -- a la Super Regulator and you can tweak up the opamps.
 

ticknpop

Member
Paid Member
2005-05-28 9:43 pm
toronto
Rasberries no way, blueberries yes.

Yes very nice parts for it's time, yes the super regulator improves it I'm sure, but the circuit topology is not the best. The AGI 511, Marantz ( class A biased the chips), and Dayton Wright SPL/SPA preamps (cascode discrete bootstrapped differential with feedforward 749 phono stage and 749 /output LM 310 current buffer line stage ) made much better use of the 739/749 ic.

Replacing the tantalums with bipolar electrolytics, renewing the power supply electrolytics, super regulating the power supply, soft recovery diodes and snubber would all help the existing unit,
But it's limited for what the circuit topology is.
 
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The IC used in the Crown IC150 line amp is totally life-less.
huhuhuhu

The LF356H still acquits itself quite well. I think you may have had an issue with the non-polarized electrolytics.

I've used NE5534, OPA627, AD825AR, LME49710, OPA604. Of these, the NE5534 is a unsung champ.

LM301 in the phono stage could probably use some freshening up...
 
Here are 2 screen shots. This is a Crown IC-150a with the stock power supply.
You can see that it runs out of gas when the output reaches 8Vrms. This is lower than specified in the manual.

Everything seems fine with the NE5534 in place of the LF356H. It tests better than the OPA627.
 

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