Steve's playback system #2: Preamp?
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- Steve E.
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Steve's playback system #2: Preamp?
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?
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?
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
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
- Steve E.
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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.
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.
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
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
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
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
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
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
- Kiss the Groove
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It's a tiny nitpick, but if you don't want to add extra resistors...
Here's another way.
Cheers -- KtG
Here's another way.
Cheers -- KtG