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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 29 Dec 2016, 14:35 
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RossD wrote:
And I think you mean a Triad C-14X rather than C-16X.
Yup. I meant the C-14X.

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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 29 Dec 2016, 17:59 
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Interesting to see Edor rate the OPTs down to 40Hz. Using the Stereophile test CD with warble tones down to 20Hz my ears told me the OPTs do 40Hz with my EL34 amp. My Black Lace, with Ecor OPTs, thought the high efficiency 15" Beymas, has its lower 3db piont at 50Hz. With still good volume at 40Hz.

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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 04 Jan 2017, 23:30 
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Joined: 03 Jan 2017, 20:59
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I am happy to read this before i complete the version 1, anyway i didn't really liked that small China made Hammond choke.
That's what i figure for the layout on a 12 x 16 inch aluminium plate with the rectifier between the choke and the power transformer.
OT are James 6135HSF 5K.
Best regards.
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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 05 Jan 2017, 17:33 
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That's the first I have heard of the China made Hammonds. Is it marked on them - made in China? I have use extensively Hammond made chokes and trannies and also OPTs.

The top plate is big enough you should have no problems assembling on that.

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Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 05 Jan 2017, 18:35 
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Joined: 20 Feb 2014, 10:35
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Location: NZ
mwhouston wrote:
That's the first I have heard of the China made Hammonds. Is it marked on them - made in China?

http://diyaudioprojects.com/Forum/viewtopic.php?f=5&t=3307


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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 07 Jan 2017, 16:00 
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Well,well,well...
I am not sure if i made a good choice for the OPT.
Of course they are probably the best for the money i paid but i want to use it mainly to drive my ESS HEIL over 1000 hz and i read big ones are best for low frequencies.


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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 07 Jan 2017, 18:16 
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lowhifi wrote:
I am not sure if i made a good choice for the OPT.
I'm not sure why you say that. The James will be good down to the lower frequencies. They are specified down to 18Hz so they should be good. They have a much larger primary inductance than the Edcor's so the trade off for very low frequency support is loss of transient response.

The interstage rolloff is fairly low (only -2/3 dB at 20Hz). See below:
Attachment:
6V6 Interstage.png
The transition frequency of the driver is also low at 2.9Hz and the power stage cathode is well bypassed, so the natural response of the transformer should dominate.

I would guess that the amp would be no more than 2 dB down at 20Hz, end to end. After it's built, let us know how the James output transformers perform.


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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 07 Jan 2017, 19:15 
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I know i am not always clear, english is not my main language.
I want to use the amp to drive these high frequency drivers over 1400 hz in an biamplified active setup.
I already drive them with a single ended 6BQ5 amp with great results.
Best regards.

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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 10 Jan 2017, 02:31 
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Matt, how do you decide what value pot to use? I see some tube amps have 100K pots and others 250K pots.

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 Post subject: Re: "Lacewood" Amp V2.0
PostPosted: 12 Jan 2017, 16:38 
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mwhouston wrote:
Matt, how do you decide what value pot to use? I see some tube amps have 100K pots and others 250K pots.
It depends on several factors.

First is the Miller capacitance of the input tube. Worst case is the entire pot value appears across the input at full volume. The high frequency cutoff is 1/(2π*Req*Cm). Since Req is the parallel combination of the driver output impedance and the volume pot, If Cm is large then I'll use a smaller control so it doesn't affect the high frequency cutoff as much if the driver has a high output impedance.

In general, the higher the gain stage, the higher Cm. And in general, the larger the tube elements the larger the Cm. High gain 6SL7 drivers have horrible CM values (140µµf is not unusual).

Second is the expected driver impedance. If is is greater than about 25kΩ then I'll try and go to a higher value pot so that I don't unduly load the driver; so long as I don't compress the high frequency response (see above).

Lastly, the lower the quality of the pot, the lower I'll go. Noise voltage is proportional the the square root of the resistor value, so cheap pots that are already noisy get lower values to try an minimize noise figure.

So there are a bunch of factors in play. I also have really come to like the 250kΩ PEC pots. They have a great feel and are very quiet (for hot formed carbon). I think it also kind of depends on my mood. ;)

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