[BUILD] BB2521~Hybrid Opamp Kit~2520 Footprint~Support Thread

[jsteiger]

Hello

This is the build support thread for the recently released BB2521. What is that you ask? The BB2521 is hybrid opamp utilizing a single DIP8 IC opamp with a pair of larger power transistors as an output stage. With this discrete output stage, the BB2521 can be used in every location that a standard 2520 can be used in. For example, the BB2521 can directly drive an output transformer (as found in a 312 preamp circuit) with excellent results. The footprint is identical to that of a 2520 to make plug-n-play in all of your favorite gear a breeze.

The original concept of the hybrid opamp is far from a new one. This circuit is heavily based on the output stage of the old B&B EQF-2 equalizer. B&B was eventually bought by Aphex somewhere around 1980. Just about every major pro audio manufacturer using chips and output transformers has used some variation of this concept including Aphex, ADM, Neve, Harrison, UREI and more. 

I have multiple kits available with a few different IC’s included. The 5534 version is a very good sounding opamp while the LME49710 version is considered sonically superior, amongst our audio friends. Feel free to experiment with other chips. Any IC that is pin compatible with a 5534 should function just fine in the BB2521. Keep in mind that a 5534 requires a 22pF cap between pins 5 & 8. These pins have no connection on the LME49710 so the cap can be left in without causing any harm.

PCB’s and complete kits can be found here.

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Classic-BB2521-Build-Guide-and-BOM.pdf
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Cheers, Jeff 

 

[RuudNL]

Just a simple question...
What is the purpose of D2 and D4? When will they conduct?
The function of D1 and D3 is clear. They provide a bias voltage for the transistors.
I'm just curious!

 

[fazer]

I'll order some in the future.  I liked the old B&B EQ's .  I believed they used 351's now a very old bifet chip. 

Price is very affordable. 

 

[jsteiger]

....Price is very affordable.

Indeed that was a primary goal here. A cost affordable alternative to a discrete opamp for those on a shoe-string budget. Plus, it is maybe a warm up for new folks to build before they tackle a discrete opamp build. To top it off, they sound very good. They may not always beat a discrete opamp sonically, but many of my friends prefer these in summing applications over their discrete counterparts.

....What is the purpose of D2 and D4?....

Well, I am no magus when it comes to discrete or IC opamp design, but I understand D2 and D4 to limit current to protect Q1 and Q2. Nearly every discrete opamp uses this config. Besides the original BB2521, they are used in the 2520, 1731, 918, 990 and so on.

 

[jimkeaney]

Found 4 of these kits on my doorstep this morning Just banged out the 1st one in about 15 minutes. Thinking I'll try these in some 500 series EQ's. Definitely easier than a GAR2520 build, but I'll post again later with my thoughts sonically.

 

[RuudNL]

I suppose they will also make a great headphone amplifier!

 

[jsteiger]

I suppose they will also make a great headphone amplifier!

Actually, I have already done this and yes it works great! This is the basis for what I am using for headphone amp duties in the modular console build.

 

[mjrippe]

Might just grab some for testing gear with 2520s.  If I swap it in for a bad 2520 I won't worry about damaging a good one!

EDIT: Just ordered, thanks!

 

[mtw]

Is this kit suitable for use with +24v 51x operation? The supply voltage range on TI's web site for the 49710 lists "Supply Voltage Range" as 2.5v to 17v and lists 36v as the "Power Supply Voltage."

It seems to me that this chip isn't suitable, but the 36v specification has me 2nd guessing myself (which I do a lot of when it comes to this kind of stuff!)

 

[shabtek]

36= +18,-18

 

[jsteiger]

Indeed, what shabtek said.

Wow, I responded this morning but the post is not there? 

 

[mtw]

shabtek: thanks for the answer and the education!

 

[metalockpick]

If you take the other two diodes out, you blow the output devices and the IC...
I found the stock config a little noisy (lots of gain, 600 Ohm load max or higher); removed the output devices, resistors and connected the IC output to the output pin. I seriously doubt it could drive headphones, but it drives an output xformer just fine, even@ max gain/level. Slightly more gain than the DOAs I tested against. I wanted to hear an OPA134; they're not ideal in the stock circuit. Anyway, easy to do, and it's one more flavor.