Hitachi SR-904

[JohnInICT]

I picked up a Hitachi SR-904 locally. It is really clean and to my surprise, sounds really good! I don't know what class G means...maybe Good. 125 watts per channel is plenty.

Anyway...needs a few bulbs. I cleaned all the pots and switches and need to adjust the bias, etc. Haven't found a service manual online...anyone have one or know the adjustments for this receiver?

Also, the bulbs are the axial type...anyone know what the ratings are?

 

[tubeboob]

Not familiar with the SR904, but I have the IA600 integrated and matching tuner.

The amplifier sounds really good, and the tuner is pretty decent as well.

I will never get rid of them, as the matching pair looks and sounds good.

Hitachi made many parts for higher end companies, so don't underestimate their quality.

Cheers

 

[JohnInICT]

I think it sounds really good. Very comparable to Pioneer, Sansui, etc. IMHO. I just need a service manual and bulbs and this thing will be golden!

 

[wizargoz]

Try here:

http://wegavision.pytalhost.com/HitachiSR-904s/

 

[JohnInICT]

You Rock! Thank you very much sir!

Try here:

http://wegavision.pytalhost.com/HitachiSR-904s/

 

[JohnInICT]

I guess the internet was wrong at the 125 watts per channel. Looks like 75 watts per channel min, 95 watts at 1 KHz and 150 watts - non clipped transient power ( I guess that means all the way up ).

 

[JohnInICT]

Read the entire manual..at least what was in English. I still don't see what type of bulbs they are. If anyone can take a look and see if I'm missing it, please do.

 

[tybrad]

Look up member dgwojo- he is the resident bulb guru/seller. He responds quicker to emails rather than PM's

 

[ecluser]

Measure the AC voltage at the two purple wires from the power transformer. I think 8V lamps should be good.

 

[JohnInICT]

I measured AC to from a ground and either lead into the lamp circuit board and got 13 - 14 volts depending on which lead

 

[ecluser]

This is not the place to measure the voltage across the lamps. Neither side of the lamps is grounded. Take the AC voltage across one lamp (doesn't matter if the lamp is good or bad), or at the ends of the purple wires from the transformer, or terminals 13 and 14 of the "DC supply board".

 

[JohnInICT]

I soldered leads onto the fuse style bulbs I was sent and they are all working great. I adjusted the DC offset. I am seriously impressed with the sound this thing puts out and it has a lot of power.

I know the service manual says 75 watts per channel min RMS at 8 ohms from 20Hz to 20KHz with no more than .09% THD

95 watts per channel @ 8 ohms/1KHz with .1% THD

I am not sure what non-clipped transient power of 150 watts per channel is...I am assuming that means wide open/all out power if it doesn't go into protection mode. If that is the case, then it is a meaningless number, right?

 

[Oilmaster]

I don't know what class G means...maybe Good. 125 watts per channel is plenty.

Class G is a very innovative amplifier class developped by Hitachi in 1976.

Quoting from Douglas Self's book:

'' This concept was introduced by Hitachi with the aim of reducing amplifier power dissipation. Musical signals have a high peak/mean ratio, spending most of the time at low levels, so internal dissipation is much reduced by running from low-voltage rails for small outputs, switching to higher-voltage rails current for larger excursions''

Followed by a circuit showing indeed two power rails (one at +/- 15Vdc and one at +/-50Vdc). And euh... 3 bias spreaders (bias generators), one for the low voltage stage, one for the positive higher voltage stage, and one for negative higher voltage rail.

So, you got yourself a pretty rare amplifier design.... fascinating

 

[SVI2004A]

Kenwood Basic M1 and M2 use that topology too

 

[ecluser]

Many manufacturers copied this principle. Nad, Yamaha and Carver for example. The Carver M-400 is just an extension of the Hitachi's invention, with three supply voltages (±25V, ±50V and ±75V) instead of 2. He pretend that he invented a "magnetic amplifier"...

 

[JohnInICT]

So, Carver copied Hitachi's design? That would explain a lot, because I thought the Hitachi sounded really close to the MXR 130 I had.

Many manufacturers copied this principle. Nad, Yamaha and Carver for example. The Carver M-400 is just an extension of the Hitachi's invention, with three supply voltages (±25V, ±50V and ±75V) instead of 2. He pretend that he invented a "magnetic amplifier"...

 

[jhoyt]

Ever since I picked up an SR-504 I've been fascinated with this series of Hitachi receivers. I now have an 804, which I think is one of the best sounding 70's pieces I've heard. And, I think they look outstanding. Congrats, I'd like to find a 904 someday!

 

[JohnInICT]

Thanks. I'm really digging it..have it all cleaned up..new bulbs..all the adjustments set...it really sounds good. I'll post some pics later.

Ever since I picked up an SR-504 I've been fascinated with this series of Hitachi receivers. I now have an 804, which I think is one of the best sounding 70's pieces I've heard. And, I think they look outstanding. Congrats, I'd like to find a 904 someday!

 

[pishta]

Just picked up a MINT example from a seller, looks like it just came out of the box, all lamps work, beutiful! Cant wait to get it home, plug in my rebuilt 1958 KLH six speakers in and tune in sime classic rock!

 

[pishta]

brought it home, set it up and it rocks! Way too nice for the garage but thats where it has to go. That's where my KLH's are anyway so.....Im stoked! I downsized from a Teac SA-200 amp, Fischer tuner, and a Technics 14 band EQ. Van Halen's "Hot for Teacher" is a great test, but mines on vinyl so its a little rumbly, but Prodigy's "Fire Starter" off CD sounds awesome!