- \n
- Phantom power voltage (P48) is 48V +\/- 4V (i.e. 44~52V)<\/li>\n
- This voltage is presented to the XLR hot and cold (pin 2 and pin 3) by 6.8k resistors<\/li>\n
- The resistors should be matched to within +\/- 0.4%<\/li>\n
- The latest spec. allows for 10mA per channel (but see below)<\/li>\n<\/ul>\n
For the remainder of this article we will assume that the phantom voltage is 48V and that the supply meets specification.\u00a0 Note that if you peruse schematics from commercial designs you may notice that the phantom supply resistors are 6k81 (i.e. 6810 Ohms rather than 6800 Ohms).\u00a0 This is because surface-mount resistors are commonly supplied as E48 or E96<\/a> values.\u00a0 The extra 10 Ohms is not important.\u00a0 What matters is that the two resistors have (ideally) exactly the same value – not that they are exactly 6800 Ohms.<\/p>\n
Here is a simplified schematic showing the basic idea of phantom power.\u00a0 Rload<\/strong> represents the power consumed by the DI box.\u00a0 The choice of Rph1<\/strong> and Rph2<\/strong> is up to the designer (but they must be identical values matched closer than 0.4% as for the source resistors).<\/p>\n
![\"\"](\"http:\/\/stompville.co.uk\/wp-content\/uploads\/2012\/03\/110SV1.png\" "\\"110SV\\"")
<\/p>\nHere is the d.c. equivalent circuit for the above:<\/p>\n
![\"\"](\"http:\/\/stompville.co.uk\/wp-content\/uploads\/2012\/03\/111SV3.png\" "\\"111SV\\"")
<\/p>\nNow effectively Rsource1<\/strong>\/Rsource2<\/strong> are in parallel and Rph1<\/strong>\/Rph2<\/strong> are in parallel. The net of two resistors of the same value in parallel is half that value so the equivalent circuit reduces to the following:<\/p>\n
![\"\"](\"http:\/\/stompville.co.uk\/wp-content\/uploads\/2012\/03\/112SV2.png\" "\\"112SV\\"")
<\/p>\nA number of things can be inferred from this schematic.<\/p>\n
- \n
- The source resistance limits the maximum current available to the load<\/li>\n
- The load is never going to see the full 48V<\/li>\n
- The lower the value of Rph<\/strong>, the more voltage and current will be available for the load<\/li>\n<\/ul>\n
If we set Rload<\/strong> and Rph<\/strong> to zero we can calculate the short circuit current at 48V \/ 3400 ohms = 14.1mA.\u00a0 However, the specification only allows for a maximum of 10mA of load current.\u00a0 48V \/ 10mA gives a total circuit resistance of 4800 ohms.\u00a0 Rsource is “fixed” at 3400 ohms, so Rph<\/strong> + Rload<\/strong> must be 4800 – 3400 = 1400 ohms.<\/p>\n
We can’t actually make Rph<\/strong> = zero because this would short out the audio signal.\u00a0 Also, the output capacitors and the phantom power resistors form a high-pass filter so, the lower the resistance of Rph<\/strong>, the more bass frequencies are rolled off, begging the question: what is the practical minimum for Rph<\/strong>?<\/p>\n
Well let’s look at it another way.\u00a0 What is the minimum useful voltage across Rload<\/strong>?\u00a0 3V? 6V? 9V?\u00a0 As stompbox merchants we’re happy with a 9V supply so let’s try that.<\/p>\n
If Rph<\/strong> + Rload<\/strong> = 1400 Ohm and current is 10mA then the voltage across Rph<\/strong> and Rload<\/strong> is 14V.\u00a0 If 9V of that 14V is dropped across Rload<\/strong> then 14 – 9 = 5V must be dropped across Rph<\/strong>.\u00a0 Therefore, Rph<\/strong> is 5V\/10mA = 500 Ohm.<\/p>\n
So – neglecting any effect on the audio signal – the lowest practical value for Rph1<\/strong> and Rph2<\/strong> is 1k0 Ohm which (when connected in parallel) gives a value of 500 Ohm for Rph<\/strong>.<\/p>\n
What about that pesky high-pass filter? Well, the corner frequency of an RC filter is 1\/(2. pi. Req. Ceq).\u00a0 We need to find values for Req and Ceq.<\/p>\n
Thevenin analysis leads us to conclude that Req is equivalent to 2x Rsource \/\/ 2x Rph and\u00a0 Ceq = C\/\/C\/\/C\/\/C.<\/p>\n
Therefore, for the above example Req = 2 x 6k8 \/\/ 2 x 1ko = 13.6k\/\/2k = 1.74k<\/p>\n
and (assuming C is 10uF) Ceq = 1\/4C or 2.5uF<\/p>\n
This gives a corner frequency of 36.5Hz, which is not so bad for a DI box.\u00a0 Also, it’s worth noting that the balanced input of most mixing desks probably have 22uF or 47uF input capacitors which serve to increase Ceq and lower the corner frequency.\u00a0 One thing we can see from this is that we need to use large coupling capacitors on the output of our DI box.<\/p>\n
So where are we up to? We have a source resistance of 3k4, a phantom resistance of 500 ohms and a load resistance which can go as low as 900 ohms.\u00a0 What happens as we vary the load resistance?<\/p>\n
![\"Load](\"http:\/\/stompville.co.uk\/wp-content\/uploads\/2012\/03\/113SV.png\" "\\"113SV\\"")
<\/p>\nPerhaps more importantly, maximum power transfer theorem predicts maximum power is available at a load resistance of 3.9k (i.e. 3.4k Rsource + 500 Ohm Rph).\u00a0 The graph of load power vs load resistance bears this out:<\/p>\n
![\"Load](\"http:\/\/stompville.co.uk\/wp-content\/uploads\/2012\/03\/114SV.png\" "\\"114SV\\"")
<\/p>\nWhat these graphs tell us is that the sweet spot (with an Rph of 500 ohms) is a load resistance of about 4k ohms.\u00a0 This allows a nice high load voltage of 24V (giving good headroom) and a generous 5mA of current.\u00a0 If we manage to design a circuit that requires less than 5mA of current then we can increase the value of Rph to raise the load voltage and thereby increase headroom.\u00a0
![\"\"](\"http:\/\/stompville.co.uk\/wp-content\/uploads\/2012\/02\/svfavicon.png\" "\\"svfavicon.png\\"")
<\/p>\n","protected":false},"excerpt":{"rendered":"I’ve been looking at the design of direct injection (DI) boxes with a view to making a phantom-powered piezo preamp for a friend who has acquired an acoustic bass with a passive piezo pickup but doesn’t have a preamp for it.\u00a0 He’s tried plugging it straight into the mixer – but has experienced major tone-suckage\u2026 Read More »<\/a><\/span><\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5],"tags":[],"_links":{"self":[{"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/148"}],"collection":[{"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=148"}],"version-history":[{"count":3,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/148\/revisions"}],"predecessor-version":[{"id":1445,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/148\/revisions\/1445"}],"wp:attachment":[{"href":"https:\/\/stompville.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=148"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=148"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/stompville.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=148"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}