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 NEW  Bruce Heran outlines the details and construction of his simple DIY 6DJ8 (ECC88) Tube Hi-Fi Headphone Amplifier Project.

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PostPosted: 13 May 2010, 14:26 
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sampleaccurate wrote:
The GXPP 10K is 2 lbs - twice the weight of the XPP at the same wattage, but with almost identical specs. I have to believe the heavier iron will behave better at low frequencies.

One main goal is keeping the cost to a minimum. Going with the upgraded trannys will push the cost over $100 which was my target.

Gio wrote:
The XPP ($17) is rated at 70 to 18K Hz and the GXPP ($26) 40 to 18K Hz. The quoted specs are IDENTICAL, yet one transformer weighs twice that of the other.

Bruce, on your push-pull EL84 tube amp builds, did you get a chance to compare the frequency response of the XPP vs the GXPP?

Gio,

Look again. If you examine the XPP line you'll notice that some are rated down to 70 Hz (that's what the description on the main page says) BUT many of the XPPs are rated down to 40 Hz including the 10 watt 10K trannys for $17.
I thought the same thing until I took a closer look at the specs. The specs on the XPP10-8-10K are IDENTICAL to the GXPP10-8-10K according to Edcor except the weight.

One thing that seems conspicuously absent from Edcors specs is the max DC bias allowable on their single ended trannys.

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PostPosted: 13 May 2010, 15:53 
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XPP10-8-10K specs:
Maximum Input: 10W
Input: 10K Ohms
Output: 8 Ohms
Frequency Response: 40~18,000 Hz, <1dBu
THD+N @ 1KHz: <0.1%
Screen Voltage: 40%
Bobbin: Glass Filled Nylon 6/6
Flammability Rate: Class B 130°
Core Type: M-6 29 ga. Grain Oriented Steel.
Termination: 0.187" (3/16") quick disconects
Mounting: Zine plated channel/frame
Unit Weight: 1.00 lbs.


GXPP10-8-10K specs:

Maximum Input: 10W
Input: 10K Ohms
Output: 8 Ohms
Frequency Response: 40~18,000 Hz, <1dBu
THD+N @ 1KHz: <0.1%
Screen Voltage: 40%
Bobbin: Glass Filled Nylon 6/6
Flammability Rate: Class B 130°
Core Type: M-6 29 ga. Grain Oriented Steel.
Termination: UL1015, 20 ga. lead wire
Mounting: Steel end bells
Unit Weight: 2.00 lbs.

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PostPosted: 13 May 2010, 16:29 
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In the UK you can still get cheap output transformers from RS (was once Radiospares) stock code 210-6475 is the current production of the RS "economy output transformer". It's rated at 3.5W though I expect you can take it well beyond that in push-pull, but probably not when used in SE mode. At less than 8 UKP it's a bargain, in quantity it's even cheaper.

See http://uk.rs-online.com


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PostPosted: 13 May 2010, 21:18 
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My guess would be that the line transformers are ungapped since they would have no DC component at their input from the driver amp. However that's speculation.

Under normal use they carry no bias current - period. A standard class A audio push-pull transformer carries bias current that heats up the copper even though the EM fields cancel so as not to saturate the core. The line transformers I'm guessing can be rated higher given the same iron and copper mass because the only signal flowing in the primary is audio (when used as directed!), not bias current to supply the plates of the tubes. If that's the case they would need to be derated for tube amp use.

That's my best guess anyway as to why the line transformers are rated so high in wattage for their size and weight.

Now why the two Edcor XPP and GXPP tansformers are rated the same with the XPP being half the weight has me stumped. Perhaps the XPP has much higher distortion - they don't spec that. It sure can't be the end bells.

I'm going to contact Edcor and ask what the difference is.

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PostPosted: 13 May 2010, 22:57 
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Hi, I did not check the XPP version as I felt it would not be likely to work as well. The GXPP10-10k-6 is a good performer. When used in the EL84 Poddwatt tube amp it will go flat to below 20HZ. Depending on the brand of tubes, the distortion in the lower region can be anywhere up to 2% at 1 watt (20HZ). I have found that the Sovtek 5751 is cleaner there than the NOS Phillips ones. With some modifications you can use 6DJ8s and 12AX7s in the circuit and they will work nearly as well. They too are brand dependent. New made Mullard 12AX7s are the best choice there. For the price the GXPPs are awfully good. I have used the CXPP25-10K-MS Edcors on the EL84s and the sound is really quite excellent, but really not so much different that the size and price are justified. This is especially true since most speakers don't really respond below about 40HZ.

Good listening
Bruce

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PostPosted: 15 May 2010, 16:09 
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Just in case you missed this, hear are frequency measurements for the Speco T-7010 audio line transformer. The results don't look bad at all considering the price. The high-end suffers more than the low. The Speco T-7010 may mate well with a bright fullrange driver.
Cheers

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PostPosted: 15 May 2010, 17:10 
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Gio wrote:
Just in case you missed this, hear are frequency measurements for the Speco T-7010 audio line transformer. The results don't look bad at all considering the price. The high-end suffers more than the low. The Speco T-7010 may mate well with a bright fullrange driver.

I missed it. Thanks. 3dB down at 20k isn't too bad. I'll give them a try. I just finished my first DIY amp last night, posting pics right now (300B tube SET).

Thanks again for the link to the Speco specs.

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PostPosted: 19 May 2010, 23:03 
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I measured the response of a Speco T-7010 tonight to get a response on the 4 ohm tap:

Image

Down almost 5 dB at 20K but less than 2 dB down at 10K. The 3 dB point I estimate to be about 13.5 kHz.

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PostPosted: 20 May 2010, 06:25 
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Good job!

In the end, we'll make a complete datasheet of T-7010. Also it will be a good thing to measure several units to stablish the "general" response. I have bought two, so I'll do it with the other one when I can.

(An autoinductance measurement is another one we need to do).

Regards!

PS: which sampling frequency did you measured?


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PostPosted: 20 May 2010, 08:54 
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Simon wrote:
In the end, we'll make a complete datasheet of T-7010. Also it will be a good thing to measure several units to stablish the "general" response. I have bought two, so I'll do it with the other one when I can.

(An autoinductance measurement is another one we need to do).

PS: which sampling frequency did you measured?

I have an 8 watt Edcor line transformer that's on order to test out that supposedly is good out to 18kHz, and they only cost $11. The primary is 10K. If the response and power handling is better than the Speco I'll probably use the Edcors.

The RTA I use (TrueRTA) first does a self calibration (48 k is the sample rate) to correct for any deviations in the audio interface or sound card from a perfectly flat response then applies that correction to the frequency sweep from a built in function generator. The bandwidth is limited to 20-20kHz, so it won't show results beyond that. I'll need a hardware function generator (on the way) and my scope if I want to go past 20 kHz in my measurements.

I also have several T-7010s. I'll test a second to see if they're consistent.

For measuring inductance of chokes and transformers I found a paper written on how to do it very simply using a regular multimeter:

http://technologyinterface.nmsu.edu/fal ... nduct.html

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