Topic: m148 bill of materials?

[Chuck]

I have been thinking about this project and really want to continue this discussion. I have attached a schematic that Jensen has on their site. Doug Oade recommended this as a good place to start. The circuit uses an AD797 op-amp. Doug suggested using an AD712 instead. I have built several op-amp based microphone pre-amps. One problem with op-amps is DC offset. Another is the need for a bi-polar power supply. Maybe we could modify this circuit to bias the op-amp for single supply operation?

[Digital Quality]

That's a single supply schematic. Am I missing something??

Some other stuff on biasing http://www.analog.com/UploadedFiles/Application_Notes/42621535592205AN581.pdf Our drive current shouldn't be very high...

[Chuck]

That's a single supply schematic. Am I missing something??

Some other stuff on biasing http://www.analog.com/UploadedFiles/Application_Notes/42621535592205AN581.pdf Our drive current shouldn't be very high...

I actually read that Analog Devices paper today. Rodney, I'm not familiar enough with transister circuits to design a pre-amp circuit with them, like Doug Oade did. You mentioned using transistors in the begining. Maybe that's the way to go. If we can build a good circuit.
I don't see on the Jensen schematic that powering the IC is addressed.

[Digital Quality]

IC or descrete stage, they are about the same except the IC takes care of getting nicely matched devices and reduces the problem of stray capacitance.

These ICs can be run in dual or single supply. The AD712 for example is a 9V to 36V device. You can use + or - voltage to come up with a combination in that range.

The ground is shown in the schematic, the positive V is not super important as long as it's at least 9v (for the AD712) and so long as the output signal is still within line level V. The next thing in the chain will then have a DC blocking cap (I believe) to take care of our biased output.

[Chuck]

The aluminum box to the left is 7.5" x 4.75" v 2". I believe that is the same size box used on the m148?
I have the rest of the stuff around from other projects. The bigger box (on the left) cost me $4.95 at a local surplus parts place. It has some holes already drilled in it, but I'll fill those and it will work great. I also think I may use the Neutrik NF2D-B-9 and NF2D-B-2 for the outputs, rather than the XLR's.

I may just go with the 6v SLA batteries after all. Building the charging circuit will be easier. Does anyone have the lowdown on the cheapest place to buy the PS605's?

Edit:
After looking at photos of the insides of the m148, I found it's the same Hammond case I have, 1590D. It turns out that my local surplus electronics store made a mistake and only charged me $4.95. That's the price without the matching top. Anyway, I'm going the start filling the holes and get it ready to drill holes in the right spots for the XLR connectors.

[Digital Quality]

Looking good!

[Chuck]

I really like the idea of using the recessed RCA jacks for the outputs.

I e-mailed Doug with some questions and told him I was going to build a microphone pre-amp similair to the M148, based on the Jensen AS018 design. He told me that the SLA cells would supply the cleanest phantom power. It's true that it would be much easier to build a charger circuit for the SLA's. He also encouraged me to use op-amps after the transformer. If I socket the op-amps, It will be possible to swap op-amps to find the best one, or provide different sonic characteristics.

[Church-Audio]

I really like the idea of using the recessed RCA jacks for the outputs.

I e-mailed Doug with some questions and told him I was going to build a microphone pre-amp similair to the M148, based on the Jensen AS018 design. He told me that the SLA cells would supply the cleanest phantom power. It's true that it would be much easier to build a charger circuit for the SLA's. He also encouraged me to use op-amps after the transformer. If I socket the op-amps, It will be possible to swap op-amps to find the best one, or provide different sonic characteristics.

Hey Chuck how much gain do you need? I would suggest for an opamp the opa2227pa and also check out the TLO82 and NE5532 they are all very good sounding opamps I see them alot in preamps. Also check out that transformer company here is a link much better prices then Jensen and half the price. http://www.sowter.co.uk/ also check these guys out I dont know what the prices are like though. http://www.stevens-billington.co.uk/index.htm These guys build very good transformers I have used the Sowter transformers and found them to be much better then the Jensens for most applications.

[F.O.Bean]

awesome chuck

[Chuck]

I really like the idea of using the recessed RCA jacks for the outputs.

I e-mailed Doug with some questions and told him I was going to build a microphone pre-amp similair to the M148, based on the Jensen AS018 design. He told me that the SLA cells would supply the cleanest phantom power. It's true that it would be much easier to build a charger circuit for the SLA's. He also encouraged me to use op-amps after the transformer. If I socket the op-amps, It will be possible to swap op-amps to find the best one, or provide different sonic characteristics.

Hey Chuck how much gain do you need? I would suggest for an opamp the opa2227pa and also check out the TLO82 and NE5532 they are all very good sounding opamps I see them alot in preamps. Also check out that transformer company here is a link much better prices then Jensen and half the price. http://www.sowter.co.uk/ also check these guys out I dont know what the prices are like though. http://www.stevens-billington.co.uk/index.htm These guys build very good transformers I have used the Sowter transformers and found them to be much better then the Jensens for most applications.

I'm aiming for 12db to 20db gain.
I am familiar with the op-amps you mentioned. The NE5532 is in a lot of gear, but I disgree with you on how it sounds. It's a good choice for driving long transmission lines, but I think there are better choices as far as sound quality. I wish I had the time/ money to audition all the various transformers out there. That, of course, would be the best way to go. The Jensens are a known quality transformer, that's why I think I'll go with those. I'll investigate the Sowters though. Cheaper is a very good thing, in my mind.

[Celac]

I'm aiming for 12db to 20db gain.
<SNIP>
 I wish I had the time/ money to audition all the various transformers out there. That, of course, would be the best way to go. The Jensens are a known quality transformer, that's why I think I'll go with those. I'll investigate the Sowters though. Cheaper is a very good thing, in my mind.

Hi,
...following your discussion with interest.  http://www.cinemag.biz/    = very similar form factor to Jensen, good reputation, and a competitve price.  Used in some pretty well respected stuff see: http://www.adesignsaudio.com/ . 
Celac.

[Chuck]

I gave those a look a while back. I wish I had a bunch of money, so I could buy them all. Run music through them and listen, to determine which is the best. The CMMI-2C on a quick look looks the closest to the Jensen JT-16-A.

Hey Celac, I don't see prices... do you know what price range these things are in?

[Church-Audio]

I really like the idea of using the recessed RCA jacks for the outputs.

I e-mailed Doug with some questions and told him I was going to build a microphone pre-amp similair to the M148, based on the Jensen AS018 design. He told me that the SLA cells would supply the cleanest phantom power. It's true that it would be much easier to build a charger circuit for the SLA's. He also encouraged me to use op-amps after the transformer. If I socket the op-amps, It will be possible to swap op-amps to find the best one, or provide different sonic characteristics.

Hey Chuck how much gain do you need? I would suggest for an opamp the opa2227pa and also check out the TLO82 and NE5532 they are all very good sounding opamps I see them alot in preamps. Also check out that transformer company here is a link much better prices then Jensen and half the price. http://www.sowter.co.uk/ also check these guys out I dont know what the prices are like though. http://www.stevens-billington.co.uk/index.htm These guys build very good transformers I have used the Sowter transformers and found them to be much better then the Jensens for most applications.

I'm aiming for 12db to 20db gain.
I am familiar with the op-amps you mentioned. The NE5532 is in a lot of gear, but I disgree with you on how it sounds. It's a good choice for driving long transmission lines, but I think there are better choices as far as sound quality. I wish I had the time/ money to audition all the various transformers out there. That, of course, would be the best way to go. The Jensens are a known quality transformer, that's why I think I'll go with those. I'll investigate the Sowters though. Cheaper is a very good thing, in my mind.

I only mention the sowters because its what Neve uses, they do not use Jensen and since Neve is the de facto standard in microphone preamps, you should take a serious look at them. I think Jensen is overrated and not nearly as good as the Sowters transformers are.

Chris Church

[Celac]

I gave those a look a while back. I wish I had a bunch of money, so I could buy them all. Run music through them and listen, to determine which is the best. The CMMI-2C on a quick look looks the closest to the Jensen JT-16-A.

Hey Celac, I don't see prices... do you know what price range these things are in?

Hi,
I called about mic split transformers and they were less expensive. I did not ask about mic input but the fellow I talked to indicated they were cost competitive across the line.  I don't think that given the value of the unit that you are home brewing a clone of the cost of the Jensen is a killer.  I would assume that the A-Designs people are using the cinemag xfrmr for its quality rather then price (given the cost of their products I'm sure they could use Jensen, Lundahl, or whoever they wished).  Neve is certainly a very respected name but not the only one.  Do the Portico pre's use Sowter?
Celac

[kskreider]

Neve is certainly a very respected name but not the only one.  Do the Portico pre's use Sowter?
Celac