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Old 11th February 2014, 11:21 AM   #1
MFF is offline MFF  Portugal
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Default 845-Amp-IT-Coupled

I would like to have some opinions about this design,
The bias of the 845 are 70V/60mA.
The drive stage bias are 2V/20mA.
I have some doubts.
what value should I use for the R8.
which IT fits better in this design.

Thanks for the help on this
MFF
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Old 11th February 2014, 02:19 PM   #2
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I would go for the Sowter 8424 because you can find its perfomance for your specific application in its datasheet. In such case use 20K for R8 (primaries in parallel). If possible, you might want to specify lower DC current as you don't need 60 mA primary DC current (primaries in parallel). 15 mA for primaries in series and 30 mA for primaries in parallel will be enough. This way primary inductance will be higher. I suppose this would be regarded as a custom order but the price shouldn't be different.
The Hashimoto could work as well but they usually recommend 1:1, 1:0.5 and 1:0.5+0.5 ratios (i.e. primaries always in series which limits the DC current to max 15 mA and 7-10 mA recommended).
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Old 11th February 2014, 03:20 PM   #3
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Quote:
Originally Posted by MFF View Post
what value should I use for the R8.
I think that if you use 47K would be worse than no gird resistor. In general, whatever value you use the result will not be great at low frequency becuase your inductance is a bit low in all cases. That's why I was telling you to get the Sowter for lower DC current rating and higher inductance.
Another (better) choice would be the Lundahl LL1692a (ALT T connection 2:3.5) that would have 35H and up to 36 mA DC current.

High gain + step up interstage for driving the 845 with low distortion are too many things for a single stage, IMHO.
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Old 12th February 2014, 09:18 AM   #4
MFF is offline MFF  Portugal
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Hi 45,
Thank you for your help!
You're right, I think the LL1692A is the best option.
You know where I can buy them.
Once again you were very helpfull.
Thanks a lot!!
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Old 12th February 2014, 10:48 AM   #5
45 is offline 45  Italy
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The closest distributor to Portugal is in Spain:
www: Granada-Audio - Válvulas y Componentes para amplificadores - Transformadores Lundahl

Otherwise ask Thomas (Vinylsavor) in this forum. Maybe he can help you.
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Old 12th February 2014, 01:54 PM   #6
MFF is offline MFF  Portugal
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Quote:
Originally Posted by 45 View Post
High gain + step up interstage for driving the 845 with low distortion are too many things for a single stage, IMHO.
I Totally agree, perhaps this configuration may help to mitigate this problem.
any suggestion?

Thanks
MFF
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Old 12th February 2014, 02:37 PM   #7
45 is offline 45  Italy
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Quote:
Originally Posted by MFF View Post
I Totally agree, perhaps this configuration may help to mitigate this problem.
any suggestion?

Thanks
MFF
How much gain do you need? Do you want an integrated amplifier with two stages?
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Old 12th February 2014, 03:29 PM   #8
MFF is offline MFF  Portugal
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I'm planning on building an 845 monoblock low voltage 845 amplifier ~5 ..15W
that can be connect to 90 db speakers.
about the gain I'd like to be able to connect direct to CD. I can also use a preamplifier(I do not know how much gain will I need)
maybe tow stages will give a better performance.
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Old 12th February 2014, 03:58 PM   #9
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I think you will get about 7W max with 610 total supply voltage at 60 mA and 5K.
If you go for the LL1692a/18mA you can experiment the connection ALT T (36 mA and 35H) with the 6C45 or the connection ALT S 4:3.5 ratio with the D3a.
The connection ALT S will allow 18 mA that are enough for the D3a with 125H primary inductance.
With the 6C45 and LL1692a ALT T you should need less than 0.7 V RMS for full power. With the D3a and LL1692a ALT S you will need about 1 V RMS. For 1W output you will need about 0.25 V RMS in the first case and 0.37 V RMS in the second. This could be enough if you use a modern CD player and 90 dB efficient speakers in most cases without adding a step-up input transformer. I would first try this two stage amp with normal volume control at the input and then if you feel the gain is not enough you can think about increasing the gain further.
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Old 14th February 2014, 10:15 AM   #10
MFF is offline MFF  Portugal
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I'll try this two stage amp, it seems to be the correct way to go... thanks!!
I'm looking at the 845 tubes, I need to buy matched pair.
I am thinking on buying TJ Full Music,
are this a good tube?
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