A DIY 1/2 Rack Bass Preamp

[Passinwind]

With the top shelf components, you'll never know till you try them. Sometimes the device application notes can be helpful in getting the most out of a part.

Yep, that was definitely true of all the frou-frou opamps I tried in this project. There are even app notes for "chip roller DIY'ers", with some good tips on stability management, and a few reality checks on real world performance gains, or not.

 

[pie_man_25]

The TL072 is probably the most widely used op-amp in the MI industry. Found in amps and

The TL072 is probably the most widely used op-amp in the MI industry. Found in amps, pedals, and mixers.

Right, I should probably have been a bit more respectful when I wrote the bit about sounding better than the tl072. I'm aware of its status, and I've got a few lying around, but I've read about a few other op-amps that apparently (subjectively) sound a bit better. Of course, the fact that I forget the names of these op-amps but remember the TL072, after seeing a lot of schematics that's got to mean something.

 

[beans-on-toast]

I would say that they use it because it has a proven track record, is inexpensive, and meets the design specs, not necessarily because it is the best performing device.

When I see these op amps in an amp's DI output for example, I tend to wish that they would have used a transformer, not a better op amp. Cost is always a consideration. Sometimes when people substitute better devices, they run into issues with oscillations so great care must be taken.

When you are designing an amp, it is alway faster to borrow ideas, layouts, etc., that are proven. No better place to start than with the TL072.

 

[Mark Reccord]

First off, another very nicely put together project, Charlie! Makes me embarrassed to post pics of the inside of my current project.

Yeah, you want to be careful when rolling opamps, especially if you're not intimately familiar with the circuitry around the devices. I've seen cases where just plopping a high performance (super wide bandwidth) device where a TL072 or NE5534 was before resulted in the new device going ballistic with HF oscillation because the power supply bypassing for the lower bandwidth parts wasn't adequate for the higher bandwidth ones.

 

[rumblinbass]

Just got the rumblinbass designed module installed and tried a quick audition once it passed bench check. It rocks, and I think I'll end using it a whole lot. I just tested a very basic config, with it in the parallel EFX loop as a clean boost that adds a little old school vibe. Should work really well as a DI driver too, I think. Thanks again for sharing some of your skills with me.

So that's pretty much what's going in the box, unless i decide to add more shielding. Still have two switches available for whatever I decide to use 'em for, wiring and detailing to tidy up, and the final front panel to install once it's made. Not a bad day's work...

Glad you like it! Coming from you, that means a lot.
Using it as a DI driver? You mean driving a transformer? Now your talkin! That's right up my alley.

 

[rumblinbass]

When I see these op amps in an amp's DI output for example, I tend to wish that they would have used a transformer, not a better op amp. Cost is always a consideration. Sometimes when people substitute better devices, they run into issues with oscillations so great care must be taken.

Transformers are expensive and there are other considerations. Op amps are more "plug and play". Both have pros and cons as with anything. Transformers are nice and do have benefits, but the cons can eat your lunch if your not careful.

 

[Passinwind]

Glad you like it! Coming from you, that means a lot.
Using it as a DI driver? You mean driving a transformer? Now your talkin! That's right up my alley.

Lucas, I have a few different Jensens mounted in outboard boxes that I'll try for starters. Could be time for some trafo rolling, I guess. Link Removed

 

[Passinwind]

Yeah, you want to be careful when rolling opamps, especially if you're not intimately familiar with the circuitry around the devices. I've seen cases where just plopping a high performance (super wide bandwidth) device where a TL072 or NE5534 was before resulted in the new device going ballistic with HF oscillation because the power supply bypassing for the lower bandwidth parts wasn't adequate for the higher bandwidth ones.

Thanks Mark. I remember when the 5534 was released, and we were putting them in all sorts of wrong places, just 'cause they were new and shiny and the spec sheet looked so winsome. I did an onboard preamp that danged near started a fire before I figured out what parasitic oscillations are all about. I stole borrowed used a pretty well proven design for my power supply in this build, and soldered in the opamp adapters rather than using sockets in those slots. The headers on the adapter are already potentially problematic -- so many rabbit holes, so little time!

 

[BassmanPaul]

As usual from our Charlie: a beautiful job!!

You must be getting close to a full house with all the suff you have built! Keep up the good work.

 

[T_Bone_TL]

I have a pile of LM833s which seem like a nice dual audio op-amp - but also may already be obsolete, but not so obsolete you can't get a few thousand from stock at Mouser (around a buck each depending on volume and variant). As always, trade-offs, different specs mattering more or less, etc, etc. And yes, those HF oscillations are always something to be on the lookout for, especially with "fancy" high slew capable op amps.

 

[beans-on-toast]

People still seem to like the 990 discrete op-amp (John Hardy $50), now based on the SSM2212 (LM394). They state that a better transistor did not improve the performance of their circuit.

 

[cableguy]

As always, nothing for sale here, just another hobby build...Link Removed

But if you had an old amp/pre you need to sell to help fund your next "hobby build" I would be OK with checking it out..........

 

[oerk]

Wow, another nice design from Charlie! And build quality / cabling is something else, I wish I was as capable of building tidy designs as you.

... capable of very manly output levels ...

I see we're using the scientific terms now

 

[Passinwind]

I see we're using the scientific terms now

OMG, OMG, I've been pwn3d. The horror!

Ahem, maximum clean output is a bit over +28dBu into an 8K ohm load. Link Removed

 

[Passinwind]

Awesome work (as always) Charlie! Looking forward to some clips.

Watch out what you ask for! Link Removed

Quick and dirty, only had a few minutes to take a first stab:

Main output, a few random EQ changes: [Invalid or Expired Link Removed]

Same clip as above, but on Soundcloud at lower-res for mobile device users, etc.:

[Invalid or Expired Link Removed]

DI output, with Jensen transformer: [Invalid or Expired Link Removed]

DI Out, no transformer, very different EQ settings: [Invalid or Expired Link Removed]

I didn't really build this preamp with recording in mind, but I think it might work out OK for me for that too occasionally.

 

[Passinwind]

Coming next: a matching 1/2 rack power amp. I hope to try out a few different modules, including some that are available to the general public. For a variety of reasons the first one won't fit in that category though, as I have been jonesing to try the new ICEpower 700ASC, and the opportunity to do so just kind of popped up out of the blue. This should come together pretty fast, will post pics soon.

 

[Foz]

Like the cat who ate cheese and sat by the mouse hole my breath is baited.

 

[Passinwind]

Like the cat who ate cheese and sat by the mouse hole my breath is baited.

That always ends well:


Link Removed

 

[agedhorse]

There are other reasons for using the TL072 op amp other than cost. There are some performance parameters that are curiously advantageous for some MI and pro audio applications.

One aspect that's valuable to explore on op amp products is what happens under heavy overload or clipping events. Very "high performance" devices may do destructively or embarrassing and unexpected things under these conditions, may have stability issues under capacitive loading (ie. some filter circuits) that require solutions that make a part perform worse than just using a more appropriate (but "lesser performance") parts regardless of what the data sheets or marketing departments might imply.

Know thy enemy before assuming one solution might be better (or worse) than another.

 

[Passinwind]

Know thy enemy before assuming one solution might be better (or worse) than another.

Absolutely, spec sheets can be hugely deceptive. Both Analog Devices and TI have some nice application notes addressing that, and I'm still absorbing that content.