Upgrade of Music Angel XD8502AIII amplifier

Part one. Entrance

Moscow firm asked me to upgrade 10 pieces of amplifiers music angel XD8502AIII.



On photo 9 amplifiers. 10 amp is already on my desktop.

Her customers complained of a dull sound.


This SE amplifier tubes DCC90+6N9P+300B





The appearance of the amplifier is very good.

The cost of these amplifiers is relatively low. In Moscow, they are sold at a price of $770

I will not criticize my Chinese colleagues, I will show only the problem areas and the possibility of improvement.

First of all, I took the amplitude-frequency response.



Here is a graph of the amplitude-frequency response



The graph shows that the reason for the "dullness" of the sound is a limited range in the high frequencies.

I have an assumption that the high frequency limitation is due to the 6N9P driver tube. This tube has a high output impedance and is probably shunted at a high frequency by a 300B tube capacity.

The second problem is due to the strong microphone effect of the DCC90 tube. This tube is unknown to me

 

Is that really a roll off of 25dB between 1kHz and 10kHz ?

 

No. I didn't sign the graph. Excuse me

The vertical scale is the voltage at a load of 4 ohms per dB (0.775V). My voltmeter has 0 dB at 0.775 v

 

Part two.Inside the amplifier

This is what the amplifier looks like inside.




Good components are installed. Most of the resistors are similar to the Allen Bradley carbon ones. Film capacitors are mostly of WIMA. Installation is neat and tough.

The technologist of the Chinese factory did a good job.

My next step will be to re-engineering

 

Part three. Drawing up a schematic diagram.

Reverse engineering has led to such an amplifier circuit.



Forecasts for improving the sound of the amplifier are very favorable.

Pleasant surprise. Electrical parameters of the tube 300B-98 produced by ELECTRON TUBE Shenguang fully comply with the electrical specifications and parameters of the tube produced by JJ Electronics.



Original 300B unfortunately I have no.

 

Part four. Modification

My upgrade plan

1. Replacement tube DСС90 double triode 6N1P
This will greatly reduce the microphone effect.


2. Replacement tube 6N9P (6N9S, 6SL7) to dual triode 6N8S (6SN7)
This will extend the frequency range on the driver tube - output tube section


3. A small correction of the scheme and the denominations of the elements.

The result of the correction:




The modes were tested on the model and it is hoped that the upgrade will effectively.

 

Hi my revision of the 300b. Referencing the valve loadlines, my version comes out pretty similar to yours.
No cathode byepass needed, as plenty of gain available. Love it.​

 

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Hi
This is my revision of the music angel. Referencing the load lines there is very little difference to yours.
No cathode byepass required,mas there is plenty of gain. Love it

 

@Wynne not seeing your images for some reason

 

My new music angel

 

hello, congratulations for this upgrade!
I'm looking for the schematic part of the power supply, specifically the inductor reference. it is defective.
I also take this opportunity to make an upgrade!

Thank you for your help