DIY line driver

[djQUAN]

I just wanted to share what I've been up to...

So a local shop I've been going to asks me if I could design and make a line driver for them that meets their needs like minimal parts in the signal path, low distortion and independent gain on all channels.

I said, give me some funding and I'll see what I can do. eight or so months later this is what we came up with.

We used PCBCart to make the PCB's, a local shop to make the the enclosure but we still haven't decided a catchy name for this thing.

I didn't have pics of the black anodized case or the copper plated one too (looks really nice) we also have an 8 channel model based on 1 and a half PCB (SMPS cut off the second board then daisy chained) and a wider chassis. other options like Fine Gold input caps and OPA2604 op amps are used on the 8ch model that the shop owner will use. this one's mine.

















some specs that I measured on my unit:
Voltage input: 0.89v - 3.69V
gain adj range: 8.5dB to 21dB
Voltage output: 9.8V RMS before clipping LED lights up
Signal to noise ratio: 110dB
Channel Separation: 100dB (measured between adjacent channels)
THD+N: 0.007%
Frequency Response:
20Hz-20kHz +0dB/-0.06dB
10Hz-100kHz +0dB/-1.2dB

[pneffkell]

I am actually searching for an 8 channel line driver. What kind of cost do you guys have on these things?

[djQUAN]

no idea on price yet. the shop owners will decide on that. I just designed the circuit and board. keep you posted when they give me the price

[tsmith1315]

Beautiful work, dj.

Who did the manual labor on PCB assembly?

It's a pleasure to see it in the forum, thanks for sharing!

[djQUAN]

Originally Posted by tsmith1315

Beautiful work, dj.

Who did the manual labor on PCB assembly?

It's a pleasure to see it in the forum, thanks for sharing!

Thanks.

I did the manual assembly. it actually isn't too hard. the most tedious part was winding the toroid. I'll be making the boards by myself filling up the orders. I just hope they don't order 50 units all at once

first time I used that Amidon toroid, I blew the switching transistors after about 40 minutes continuous operation because I was using a too high frequency causing too much core losses. didn't realize something was wrong because it worked and measured fine within a few minutes of running while the core was cold.

the shop owner was planning to keep the "High End" theme so they will come stock with BB OPA2604 op amps, Nichicon Fine Gold input caps and Panasonic FM PS caps. he does want something better for the trimmers. any substitutes to the Bourns that is better but fits the footprint?

[Perry Babin]

I can say that you're far better than I am at SMD assembly. I can never get the solder paste that consistent.

Increasing the number of turns will also reduce heating. For testing in the future, connect a milliamp meter in series with the B+ supply for the line driver and use a heat gun to heat the core. If the increase in current is significant, you know that you need to make changes. If you can find the P-material cores in the size you need, they may be a good choice. The loss is at its lowest at about the maximum operating temperatures you find in vehicles.

[djQUAN]

Thanks, I did not use solder paste though. each one is placed and soldered using 63/37 leaded solder wire. takes me around 2 hours or so to complete a board when all parts are at hand. talk about hand crafted

I used 77 material core from Amidon. FT-63A-77

20+20T for primary and 26+26T for secondary. haven't done much tests if it is optimum though. before, it was 15+15 and 18+18 and freq is 57kHz and the core heated too much from core losses and blew the switchers. dropped it to 38kHz and the waveforms are perfect square waves with no ringing/overshoot and no heating. will try the heatgun trick and further optimize the toroid when I dismantle it again to replace parts with Neutrik RCA's, BB op amps and Panasonic FM caps.

[theAnonymous1]

Why not a buffered output stage? BUF634 or LME49600?

Between the cable capacitance and the fact sometimes a few parallel inputs are driven; it seems like good idea, if not a necessity, for a proper car audio line driver.

[richie00boy]

The chip used will happily drive a few 10k amp loads.

Are you using regulation on the TL494?

[djQUAN]

Originally Posted by theAnonymous1

Why not a buffered output stage? BUF634 or LME49600?

Between the cable capacitance and the fact sometimes a few parallel inputs are driven; it seems like good idea, if not a necessity, for a proper car audio line driver.

we chose OPA2604 for the high voltage and high current capability. BUF634 will increase performance further but will add complexity and more parts in the signal path. the customers wanted as few parts in the signal path as possible.

Originally Posted by richie00boy

The chip used will happily drive a few 10k amp loads.

Are you using regulation on the TL494?

nope. it's open loop. I only used the oscillator and soft start function of the TL494. not enough board area to do a regulated PS unless going with more specialized controllers. seems to work just fine