[dcheming]

I recently got my two turntables out of storage after three years. Since I gave my old DJ mixer to a friend a while back I needed a cheap solution for a phono preamp. Eventually I want to build my own mixer but for the time being I chose to simply build the $15 Velleman K2573 Stereo RIAA Correction Amplifier.

I substituted some of the low-grade parts included in the kit with the best I could find at Norvac Electronics: the signal caps are mylar bypassed with Dayton film/foils instead of the stock polar electrolytics, the equalization capacitors are silver/mica instead of the stock ceramics, and all the resistors are NTE metal oxide.

The sound of this is actually pretty good surprisingly. Overall I like it better than my friends slightly modified NAD PP-1. The only area the NAD beats it is the deep bass extension but we're planning on putting polypropylene caps in the NAD soon though which should turn the tables. Next up will be a Bugle phono preamp from Hagtech.

 

[margolane10]

That looks great! I'm getting into vinyl and need a preamp. What was the difficulty level of this project? Do you think a complete beginner like me would be able to build one?

 

[nikongod]

can you remove the input caps?

why no ground point? does your TT send the tonearm/shield ground out in the signal ground?

 

[dcheming]

Quote:

Originally Posted by margolane10 /img/forum/go_quote.gif That looks great! I'm getting into vinyl and need a preamp. What was the difficulty level of this project? Do you think a complete beginner like me would be able to build one?

Thanks. This is definitely a basic project to assemble so how easy it will be really depends on if you can already solder or not. If not then you can always practice soldering and desoldering on a broken VCR or something. Also the kit only includes the circuit board and the parts that go on it. The box, RCA connectors, toggle switch, power connector, stand-offs, diode, and rubber feet where all purchased separately. You'll also need a 10 to 30 VDC wall-wart.


Quote:

Originally Posted by nikongod /img/forum/go_quote.gif can you remove the input caps?

I'm not sure on this. I would think that you could since I don't see how any DC could be coming from a cartridge, but I could be wrong.


Quote:

Originally Posted by nikongod /img/forum/go_quote.gif why no ground point? does your TT send the tonearm/shield ground out in the signal ground?

Honestly I completely forgot about including a ground post when I was visualizing this project. I didn't even realize this until I went to hook my 'table into it.

 

[luvdunhill]

Quote:

Originally Posted by dcheming /img/forum/go_quote.gif I'm not sure on this. I would think that you could since I don't see how any DC could be coming from a cartridge, but I could be wrong.

probably needed for moving magnet cartridges. Capacitance is used to load MM carts like resistance is used to load MC carts.

 

[Sarchi]

Quote:

Originally Posted by dcheming /img/forum/go_quote.gif Next up will be a Bugle phono preamp from Hagtech.

The Bugle is a very good sounding phono, it compares well against the Lehmann Black Cube which costs several times more. I've been using the Bugle for close to 2 yrs and it has 'survived' a few upgrades to my front end (turntable, then cartridge). It also works really well with the Denon DL-103 and Lundahl LL9206 stepup trannies.

I fried one of the op amps but I think it was my fault, forgot to switch it off while I was disconnecting the battery leads. I use Panasonic 12V SLA batteries.

Nice job on the build, btw.

 

[nikongod]

i forgot to say before and meant to:
the case work does indeed look very nice. it looks like there is ample room for some other stuff should you ever decide to add it (a moving coil stage for example.)
Quote:

Originally Posted by dcheming /img/forum/go_quote.gif I'm not sure on this. I would think that you could since I don't see how any DC could be coming from a cartridge, but I could be wrong.

some op-amps do "more or less" require an input cap (op-amps with bipolar input transistors for example) but you may even still be able to get away without one on a phono stage, where it looks like there is an output cap.
Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif probably needed for moving magnet cartridges. Capacitance is used to load MM carts like resistance is used to load MC carts.

the cap for loading an MM cart is usually TINNNY (like a few pF as opposed to uF) and in parallel to the 47kohm loading resistor. im pretty sure that cap is there as a DC blocking cap.

 

[amb]

Depending on the opamp used, input coupling caps (in series with the input signal) may be advisable to prevent DC bias current from going through the cartridge coils.

The loading caps (going from inputs to ground, in parallel with the 47K resistor) for MM cartridges depend heavily on the cartridge itself. The ideal value varies, and the tone arm cable/interconnect cable from the turntable cable capacitance must also be taken into consideration. The total value is typically in the several hundred pF range. Too little capacitance and you'll have a peaky HF response, too much and the HF response will roll off.

 

[JensJ]

Quote:

Originally Posted by dcheming /img/forum/go_quote.gif I recently got my two turntables out of storage after three years. Since I gave my old DJ mixer to a friend a while back I needed a cheap solution for a phono preamp. Eventually I want to build my own mixer but for the time being I chose to simply build the $15 Velleman K2573 Stereo RIAA Correction Amplifier. I substituted some of the low-grade parts included in the kit with the best I could find at Norvac Electronics: the signal caps are mylar bypassed with Dayton film/foils instead of the stock polar electrolytics, the equalization capacitors are silver/mica instead of the stock ceramics, and all the resistors are NTE metal oxide. The sound of this is actually pretty good surprisingly. Overall I like it better than my friends slightly modified NAD PP-1. The only area the NAD beats it is the deep bass extension but we're planning on putting polypropylene caps in the NAD soon though which should turn the tables. Next up will be a Bugle phono preamp from Hagtech.

Well if you do a spice-analysis, the amplifier has more than 3 dB deviation from the RIAA curve. The response peaks at 10 kHz and has its lowest value around 110 Hz (more than 3 dB), so no wonder there is too little bass.
Component values should be: (only one channel shown)
Unchanged:
R5 = 1M
R6 = 2.2k
R8 = 3,3k
Change:
R4 from 100k to 76.33k
C4 from 680pF to 1.06NF
C5 from 3.3n to 2.95nF
This will give a response within +/- 0.1 dB. You will have to find suitable parallel combinations for this yourself.
In my experience it is almost impossible to make a phono preamp with standard component values.

 

[dcheming]

Quote:

Originally Posted by JensJ /img/forum/go_quote.gif Well if you do a spice-analysis, the amplifier has more than 3 dB deviation from the RIAA curve. The response peaks at 10 kHz and has its lowest value around 110 Hz (more than 3 dB), so no wonder there is too little bass. Component values should be: (only one channel shown) Unchanged: R5 = 1M R6 = 2.2k R8 = 3,3k Change: R4 from 100k to 76.33k C4 from 680pF to 1.06NF C5 from 3.3n to 2.95nF This will give a response within +/- 0.1 dB. You will have to find suitable parallel combinations for this yourself. In my experience it is almost impossible to make a phono preamp with standard component values.

Thanks for running SPICE on this circuit. Next time I go to the parts store I'll pick up the suitable components to build the values you've listed and post my impressions.

 

[JensJ]

I used these values from the E12 series in the analysis:
R4 82k parallel with 1M2.
C4 1nF parallel with 33pF.
C5 2n7 parallel with 220pF.
The values incorporate a correction for limited loop gain at higher frequencies.
Tolerances should be 2% or better, but 5% "will do".
Good luck!

 

[heatmizer]

Along these lines does anyone like the VSPS.
RJM Audio - The Very Simple Phono Stage

 

[Uncle Erik]

Nice work!

Norvac brings back a few memories - I used to regularly drop in there when I lived in Salem.

 

[Kirnupiima]

Hello!
Are there any news on the DIY phono preamp world? I'm itching for a new project and one of these could very well be it!
I'm looking to upgrade a Pro-ject Phono Box II into something more special and self-made.

 

[dcheming]

  Hello!
Are there any news on the DIY phono preamp world? I'm itching for a new project and one of these could very well be it!
I'm looking to upgrade a Pro-ject Phono Box II into something more special and self-made.

 
Something that I've been looking at is the Twisted Pear Audio Retro phono stage: http://www.twistedpearaudio.com/linestages/retro.aspx
 
I'm interested in it because my rig is fully balanced these days.