Steve's playback system #2: Preamp?

Post: # 14416Unread post **Steve E.** Wed Apr 20, 2011 7:59 pm

I would love to use my Presto 6N as my primary playback turntable, at least for now, as I am on a tight budget. It has a 12" balanced homemade tonearm created by its previous owner out of some sort of copper pipe (!), which I am finding tracks remarkably well, with as little distortion as I have encountered in any turntable of mine. There is no antiskating, so it is a little primitive, but honestly, it sounds great.

So....I'd like to upgrade the quality of the thing over time, and I am fishing for suggestions in a series of threads.

2) It occurs to me that I might want to invest in a stereo phono preamp. As I play both 78s and 33's, I'd like a preamp that at least allows me to turn OFF the RIAA curve altogether, play 78s, and adjust my own EQ. I am on a VERY tight budget. Any suggestions?

Post: # 14423Unread post **markrob** Wed Apr 20, 2011 9:31 pm

Hi,

Best bet would be to buy a standard phono pre-amp and remove the RIAA network to make it flat. You typically need a gain of about 100 (40db) to bring most magnetic pickups up to line level. Here is a link to a decent inexpensive unit. I have one of these and it uses through hole parts, so its easy to work on. I use mine with the RIAA (I have a home built flat preamp as well), but I can look inside mine, trace out the circuit, and let you know what mods are needed to make it flat.

http://pro-audio.musiciansfriend.com/product/BBE-FJB200X-Phono-Preamp?sku=801600

As far as non-RIAA playback goes, you can use an external EQ or if you are interested, I can send you my VST plugin that has adjustable turnovers so that you can setup presets for any of the older curves.

Mark

Post: # 14429Unread post **Steve E.** Thu Apr 21, 2011 12:44 am

Ha! That's the one I almost bought yesterday! I looked inside it but had no idea how to mod it.

It's wonderfully cheap! very tempting. Yes, I'd love to know how one would mod it. Rather have it start out flat than undo the EQ externally. But please don't spend too long unless you are curious about it yourself.

There are kits out there too....I'm just being lazy and not fully exploring them.

Post: # 14452Unread post **markrob** Thu Apr 21, 2011 1:02 pm

Hi Steve,

If this is what the current PCB looks like, its easy to change the preamp to flat

http://home.comcast.net/~markrob1066/pwpimages/BBE_Preamp.JPG

I traced the schematic for the preamp:

http://home.comcast.net/~markrob1066/docs/BBE_Schematic.pdf

To make the preamp flat remove the two 820pf caps marked C8 and C10. Then short out the two 3300pf caps marked C7 and C9.

The other way to do the mod would be to remove the two 3300pf caps and tack solder in parallel or replace the two 1meg resistors marked R7 and R10 with a 100K units.

This will leave you with a flat gain of about 100.

Mark

Post: # 14454Unread post **Steve E.** Thu Apr 21, 2011 3:35 pm

WOW. thank you.

I am always amazed at this stuff. It's like magic that you can do that.

Is it possible to add a switch to engage the RIAA, or does that make it much more complicated/impossible?

Post: # 14457Unread post **markrob** Thu Apr 21, 2011 5:08 pm

Hi Steve,

Here is the schematic modified to add a DPDT switch to allow you to operate flat or with RIAA.

http://home.comcast.net/~markrob1066/docs/BBE_Schematic_Mod.pdf

Also note, the correction to the values of R6 and R11. I had them as 1.1K. They are really 1K. A good additional mod would be to use a better opamp (I like the OPA2604). I'd also consider reducing the value of R12 and R13 to zero or a much lower value like 100 ohms.

Mark

Post: # 14482Unread post **markrob** Fri Apr 22, 2011 10:56 am

Hi Steve,

I made a minor change to the schematic for the switchable option to avoid a nasty pop when the switch is operated. If you previously downloaded the mod, get a new version at the same link. If I get a chance, I'll modify my unit and let you know if there are any other issues.

Mark

Post: # 14723Unread post **Steve E.** Mon May 02, 2011 4:59 pm

thank you!

Post: # 14782Unread post **markrob** Fri May 06, 2011 11:45 am

Hi Steve,

I did make the mod I proposed to my BBE preamp and it works fine. It can now can be swithed to either RIAA or flat. I only had a DPDT on-off-on switch in my junk box (you really want an on-on type), so I have three positions. RIAA, Flat 10X , Flat 100X. I also added a socket so that I could use other OP-AMP's (currently using an OPA2604). Also, note on the modified version of the schematic R15 and R16 (100K) are not on the PCB. These need to be tack soldered to the board on one end and wired to the switch on the other end. If you are interested, I can post a picture of the completed mod.

Mark

Post: # 15330Unread post **Steve E.** Mon Jun 13, 2011 4:49 pm

Doy! How did I skip that? I'd Looooove to see the completed version.

Post: # 18166Unread post **Steve E.** Mon Feb 13, 2012 11:16 pm

Does anyone have any experience with the Hagerman/Hagtech Bugle preamp kit?

Post: # 18180Unread post **Kiss the Groove** Tue Feb 14, 2012 12:12 pm

It's a tiny nitpick, but if you don't want to add extra resistors...

Here's another way.

Cheers -- KtG

Post: # 18314Unread post **markrob** Thu Feb 23, 2012 12:09 pm

Hi Steve,

Here are some quick shots of the BBE Preamp mod.

Hope that helps.

Mark

Post: # 18315Unread post **Steve E.** Thu Feb 23, 2012 12:21 pm

Nice! thank you!