Massdrop x Eddie Current ZDT Jr. (Official Thread)

So it looks like the 6ZH1P driver tube is retained and a hardware solid-state circuit replaces the power tube. Eagerly awating more details

 

Quite so.

I took from the manufacturer ZDT-Jr. I was not given a serviceable amplifier. Serviceable amplifiers they are in reserve for warranty cases and do not give anyone.
I will need time to repair this amplifier and test it in more detail along with the adapter.

 

Pardon me is the 2 tubes per side now become 1 tube after the modification? I always wish the NMB has better bass and very looking forward to this mod.

 

I tried to remove the nuts, but my sockets don't catch, their threads are a little recessed and the nuts are very shallow. So I couldn't remove the power transformer cover

So, instead I wrapped a few turns of Zampotech's transformer shield around both output transformers.

Think it helped, before this, I could hear hum when I connected a pair of Lyr II IEMs, but after the hum was really, really low. Plan to test a little more, this week. I want to remove the output tx covers so its easy to take on and off the tx shielding, and do a quick comparison.

BTW, with my HD800's, hum is really low before I added the shield, only an issue with IEMs.

Randy

 

There is another beneficial side effect. The screens around the output transformers expand the soundstage a bit

 

The result of comparative tests of the adapter.



1. THD measurement

1.1 THD at output load 100 ohm and output signal level 0 dB

ZDT Jr original


ZDT Jr + adapter


1.2 THD at output load 100 ohm and output signal level -10 dB

ZDT Jr original


ZDT Jr + adapter


The adapter reduces the THD by about half.


2. Measurement and calculation of output resistance

Measurements were performed at a frequency of 1 kHz and an output level of 300 mV (-8.5 dB)

2.1 Output impedance of the output is Low

ZDT Jr original = 19 ohm
ZDT Jr + adapter = 6.5 ohm

2.2 Output impedance of the output is High

ZDT Jr original = 40 ohm
ZDT Jr + adapter = 13 ohm

3. Frequency response
The frequency response in both cases was identical. But an amplifier with an adapter allowed lower frequencies to be transmitted with greater amplitude and less distortion.

Perhaps the best low-frequency transmission and low output impedance of the amplifier with the adapter created the "bass monster" effect

 

This sounds very much like ECP T3/T4 amps. Tube -> Transistor -> Transformer.

Seems like a very interesting mod

 

Sorry for not knowing. Tell me, what is this amplifier?

A Google search did not yield anything.

 

http://www.diyforums.org/Torpedo-III/TorpedoIII-overview.php

 

Sorry if I'm being dense, but this confuses me. If the frequency response of the original and +adapter are the same, then shouldn't the relative amplitudes across the frequency range be the same? Can you show us graphically how the lower frequencies had greater amplitude with the adapter?

Thanks!

 

Unfortunately, I won't be able to photograph the oscilloscope screen at a very low scan rate.
I'll explain in words:
As long As the output voltage is up to 1.2 V at 21 Hz, both options behave the same way
If you continue to increase the output level, ZDT-Jr begins to distort the signal due to saturation of the magnetic circuit transformers. ZDT-Jr+adapter begins to distort the signal only after the output voltage of 1.7 v or more

In both cases the High output and the 100 Ohm load are used

 

Is a ZDT Jr purchased in the September 2019 drop a V1 or V2 one? Serial 0161

 

I guess the second drop went live in July 2019. So it must be the V2 if the person got it from drop.
However I must say there is no V1 or V2 per se. It's just that its the second batch of amp after 1st drop I guess with low hum

 

Pretty sure all v2 models have the white dot next to the serial number. They have the hum fix.

**edit... managed to type ‘whore dot’...

 

Something like this dot?
I got it last year in July from drop. I wouldn't say no hum but very feeble which doesn't bother me at all.

 

Yes sir that is the dot! Should be a v2

 

Had one of these amps in the past, "unlucky" #13, and missed it so I purchased one online that was broken. Ended up only being a broken fuse, but the amp does have the hum issue. Does anyone have any extra steel from when they modified their unit?

 

As a non-practicing (aka forgotten almost everything) electrical engineer, I wanted to double check if the polarity for the toroidal transformer connections matter in this circuit.

Getting ready to cut the wires to move the center transformer. Afraid of pulling the hot-glue as the globs are pretty huge so am thinking of patching in an inch or two of wire. Any other recommendations for this process?

Thanks!

 

If you want to change the polarity, you can do it without soldering, changing the polarity on the power connector. Sometimes, after changing the polarity, the amount of interference coming from the power supply changes.

If you want to solder in glue:
Stick the hot tip of the soldering iron directly into the glue. It melts well and is easily removed when soft.

 

I screwed up causing a short in the transformer output on the bridge rectifier. I think I fixed the short now, but the damage is done (not working argh).

Does anyone know what the transformer output voltages should be?

I am seeing about 1.7V across the capacitor in front of the diodes. The other two capacitors are ~200V.

Any other recommendations for debugging. Thanks!