ProtoDAC TDA1387 X8 project

[thomaschan]

The "FET-ishizator"? Quite a sense of humor.

That is passive I/V with a simple FET inverting gain stage. The 90 ohm resistor is the passive I/V resistor. The FET VAS gets you probably the 12-13 dB of gain for line level. This will also get you polarity inversion, like the other Lampizator circuits. If you are using SPDIF input, this will invert polarity to normal. The BF245B is discontinued. I was just looking at the BF245A with another DAC I/V, and that is discontinued also. I see they have them on Ali.

Have you tried it without the 220uF cap on the 820R source resistor? That will give some local feedback and lower distortion. It may reduce the gain too much.

It sounds like TDA1387 X8 plus passive I/V plus FET VAS is a winner. Thank you for sharing and posting your findings, and the detailed picture of your streamer.

For true active I/V conversion, the gate of the FET, grid of the tube, or noninverting input of the op amp need to be grounded or at a voltage referenced level.

One interesting active I/V circuit I just found is the D1NOB1 in this diyA thread. It is balanced, but just use half of the circuit for single ended. It is a mod of the Pass D1 I/V stage. There is a version with a buffer output (like the Pass B1 preamp) called the D1B1 in the same thread, but performance is worse, so they dumped the buffer. I think this could be easily adapted to the TDA1387 X8 as is. 

If you reduce it to a single-ended version, it is only slightly more complex than the FET circuit above. They later upped supply voltage to +45V/-45V to get ridiculously low THD numbers (-130dB). Power dissipation was quite high though , requiring significant heat sinks. Then in the later posts, OPC realized you could do away with the -45V supply and still have excellent performance with reduced power dissipation.

The TDA1387 X8 would see very low resistance with the D1. Much lower than passive I/V. I am not shure if the TDA1387 X8 really cares. Once you get below 100 ohms, it may not matter.

I did not put in any cap in parallel to the 820 ohm resistor. The sound quality and gain is quite OK to me. I choose this FET design as opposed to tube because I want to put everything together in a not too big chassis. The FET output only requires DC15V which takes up very small space. 

I built this for an old friend together with a preamp (based on MBL6010) and small mono block LM1875 power amp. He will be using B&W 606S2 speaker. Should be quite decent setup for sweet vocal.