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Nobsound Tube Preamp

meako

meako

New Member
I recently acquired this preamp from Nobsound/Douk Audio. It is based on the Marantz 7 line stage with the Shigeru Wada circuit mod. It had two 12ax7s and two 12au7s from PSVANE.

Stock tubes sounded awful. It was very metallic and the bass was really weak. So, I ended up replacing the 12au7s with NOS National 5814a tubes. Then I replaced the 12ax7s with the stock 12au7s. This preamp now sounds very airy with strings and guitars seemingly popping out of my speakers. The bass is now well bodied and warm.

I'm currently waiting on a pair of 5814a to replace the 12au7s. Hopefully this will make this preamp sound even better.

Btw, I also replaced the stock knobs with aluminum ones from Amazon. I like the retro minimalist look :)
 

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@meako , that is one of at least two models of the Marantz 7 that zerozone makes. The other is fully remote controlled (see attached photo). Somewhere on this site, there is a whole series of threads on it.
 

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mcgjohn said:
@meako , that is one of at least two models of the Marantz 7 that zerozone makes. The other is fully remote controlled (see attached photo). Somewhere on this site, there is a whole series of threads on it.
Click to expand...
Those Marantz 7 clones by zero zone I believe, does not have the Shigeru Wada circuit mod. I was looking into them also before I decided on the Nobsound. They seem to be the same as the ones sold at Tube4Hifi. Those also are almost double the price of the Nobsound it seems.
 
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meako said:
Btw, I also replaced the stock knobs with aluminum once from Amazon. I like the retro minimalist look :)
Click to expand...

Looks very interesting and very nice visual upgrade from the black plastic with those knobs! It's a little odd to me that it doesn't have any branding or labels anywhere on it (unlike other Nobsound products), but I guess it adds to the "minimalist" look.
 
This looks like the one I reviewed: There is no safety ground, which is dangerous for anything with a metal chassis, it uses a 110V transformer, which in the US on 120V has the tube heaters running at over 7V, has really poor quality input jacks, tube sockets and other components, some of which fell apart in my hands. Sonically, even with good tubes, it was at the bottom of the china pre-amp shootout I did and also provided almost no gain.

 
At the very leased, fix that safety ground and reduce those high heater voltages.
If you keep running the heater voltages that high, those tubes are not going to last near as long as they should.
 
StepheK said:
This looks like the one I reviewed: There is no safety ground, which is dangerous for anything with a metal chassis, it uses a 110V transformer, which in the US on 120V has the tube heaters running at over 7V, has really poor quality input jacks, tube sockets and other components, some of which fell apart in my hands. Sonically, even with good tubes, it was at the bottom of the china pre-amp shootout I did and also provided almost no gain.

Click to expand...
I really enjoyed your review. The only thing missing there was an actually test on how it sounds playing music. With the 12au7s replaced by 5814a tubes and the 12ax7s replaced by the 12au7s, this preamp compares very well with the preamp section of my NAD 3130, 7140 and 7150. The NADs except for the 7150, has a darker warmth. This preamp though flat out beats the NAD 7150, with the current tube configuration I have. The highs are very clear and sweet and the lows are warm and tight. Its music resolution is, in what I can hear, is above my NADs.

Imo, this is really a steal. You have a nice looking amp with 4 inputs. It is a fully tube preamp with tube rectification. It has point to point soldering except for the power supply section.

Concerning though are the problems you pointed out. The heaters and grounding. Shouldn't grounding be an easy fix? I just have to connect the ground of the power plug port to the chassis right?
 
This metal connector is the one I need to connect to the chassis to ground the preamp right? I remember from the video that the inputs are already properly connected for ground to the chassis.
 

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StepheK said:
There is no safety ground, which is dangerous for anything with a metal chassis...QUOTE]
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Finally, I was able to ground the amp to chassis using a pice of copper electrical wire. Is this enough to fix the lack of ground issue? Thanks again for your many advice on your YouTube channel.
 

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meako said:
Finally, I was able to ground the amp to chassis using a pice of copper electrical wire. Is this enough to fix the lack of ground issue? Thanks again for your many advice on your YouTube channel.
Click to expand...

That will fix it.
 
meako said:
I recently acquired this preamp from Nobsound/Douk Audio. It is based on the Marantz 7 line stage with the Shigeru Wada circuit mod. It had two 12ax7s and two 12au7s from PSVANE.

Stock tubes sounded awful. It was very metallic and the bass was really weak. So, I ended up replacing the 12au7s with NOS National 5814a tubes. Then I replaced the 12ax7s with the stock 12au7s. This preamp now sounds very airy with strings and guitars seemingly popping out of my speakers. The bass is now well bodied and warm.

I'm currently waiting on a pair of 5814a to replace the 12au7s. Hopefully this will make this preamp sound even better.

Btw, I also replaced the stock knobs with aluminum ones from Amazon. I like the retro minimalist look :)
Click to expand...
Have you measured the heater voltages? Until they're running at the correct voltage I would suggest that you just leave the stock tubes, which are junk, in place.

Also, if the preamp is designed to use a 12AX7 as the input tube, changing it to a 12AU7 totally changes the circuit.

The two tubes use different operating points. While some preamps are designed so that a variety of 12A*7 family tubes can be used, if it's designed to use a 12AX7 all bets are off.

Perhaps you could measure the voltages with a 12AX7 in place and then with a 12AU7 and post them.
 
Yo
FlaCharlie said:
Have you measured the heater voltages? Until they're running at the correct voltage I would suggest that you just leave the stock tubes, which are junk, in place.

Also, if the preamp is designed to use a 12AX7 as the input tube, changing it to a 12AU7 totally changes the circuit.

The two tubes use different operating points. While some preamps are designed so that a variety of 12A*7 family tubes can be used, if it's designed to use a 12AX7 all bets are off.

Perhaps you could measure the voltages with a 12AX7 in place and then with a 12AU7 and post them.
Click to expand...
You are correct Charlie. Changing the 12ax7s with 5814s gave me a weaker bass and the harsh highs got tamed but not enough. Will a 5751 be a good replacement for the 12ax7s instead? Also would having all mil spec tubes be better against the higher voltages? Or is it just better to buy a step down transformer? Thanks.
 
Military spec tubes might be more rugged in some ways but their heater voltage still needs to be 6.3v. You could use a bucking transformer or add some low value resistors to the heater circuit.

A 5751 would probably be the only tube I'd want to use as a possible sub.
 
StepheK said:
This looks like the one I reviewed: There is no safety ground, which is dangerous for anything with a metal chassis, it uses a 110V transformer, which in the US on 120V has the tube heaters running at over 7V, has really poor quality input jacks, tube sockets and other components, some of which fell apart in my hands. Sonically, even with good tubes, it was at the bottom of the china pre-amp shootout I did and also provided almost no gain.

Click to expand...

FWIW I had this preamp on order, but after watching this video, I canceled the order.
 
FlaCharlie said:
Military spec tubes might be more rugged in some ways but their heater voltage still needs to be 6.3v. You could use a bucking transformer or add some low value resistors to the heater circuit.

A 5751 would probably be the only tube I'd want to use as a possible sub.
Click to expand...
I'm hunting right now for some good but affordable 5751s. Any recommendations? Btw, is this transformer ok to turn down the voltage to 110v? Thanks again.
Yinleader 500W Voltage Transformer Power Converter(220V to 110V, 110V to 220V) Step Up/Down Converter 110/120 Volt - 220/240 Volt https://www.amazon.com/dp/B07PS3NKWS/ref=cm_sw_r_apan_i_9TJAYNNY9RRJHYF80G1X
 
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